diff --git a/src/nfa/castlecompile.cpp b/src/nfa/castlecompile.cpp
index 4bddf767..11ae2000 100644
--- a/src/nfa/castlecompile.cpp
+++ b/src/nfa/castlecompile.cpp
@@ -904,7 +904,7 @@ void addToHolder(NGHolder &g, u32 top, const PureRepeat &pr) {
     if (min_bound == 0) { // Vacuous case, we can only do this once.
         assert(!edge(g.start, g.accept, g).second);
         NFAEdge e = add_edge(g.start, g.accept, g).first;
-        g[e].top = top;
+        g[e].tops.insert(top);
         g[u].reports.insert(pr.reports.begin(), pr.reports.end());
         min_bound = 1;
     }
@@ -914,7 +914,7 @@ void addToHolder(NGHolder &g, u32 top, const PureRepeat &pr) {
         g[v].char_reach = pr.reach;
         NFAEdge e = add_edge(u, v, g).first;
         if (u == g.start) {
-            g[e].top = top;
+            g[e].tops.insert(top);
         }
         u = v;
     }
@@ -933,7 +933,7 @@ void addToHolder(NGHolder &g, u32 top, const PureRepeat &pr) {
             }
             NFAEdge e = add_edge(u, v, g).first;
             if (u == g.start) {
-                g[e].top = top;
+                g[e].tops.insert(top);
             }
             u = v;
         }
diff --git a/src/nfagraph/ng_dump.cpp b/src/nfagraph/ng_dump.cpp
index 57668caf..7c1894a3 100644
--- a/src/nfagraph/ng_dump.cpp
+++ b/src/nfagraph/ng_dump.cpp
@@ -234,9 +234,9 @@ public:
     void operator()(ostream& os, const EdgeT& e) const {
         // Edge label. Print priority.
         os << "[fontsize=9,label=\"";
-        // If it's an edge from start, print top id.
-        if (is_any_start(source(e, g), g) && !is_any_start(target(e, g), g)) {
-            os << "TOP " << g[e].top << "\\n";
+        // print tops if any set.
+        if (!g[e].tops.empty()) {
+            os << "TOP " << as_string_list(g[e].tops) << "\\n";
         }
 
         // If it's an assert vertex, then display its info.
diff --git a/src/nfagraph/ng_equivalence.cpp b/src/nfagraph/ng_equivalence.cpp
index d0ab7c4a..383b6c75 100644
--- a/src/nfagraph/ng_equivalence.cpp
+++ b/src/nfagraph/ng_equivalence.cpp
@@ -72,7 +72,7 @@ struct VertexInfoPtrCmp {
 class VertexInfo {
 public:
     VertexInfo(NFAVertex v_in, const NGHolder &g)
-        : v(v_in), vert_index(g[v].index), cr(g[v].char_reach), edge_top(~0),
+        : v(v_in), vert_index(g[v].index), cr(g[v].char_reach),
           equivalence_class(~0), vertex_flags(g[v].assert_flags) {}
 
     flat_set<VertexInfo *, VertexInfoPtrCmp> pred; //!< predecessors of this vertex
@@ -82,7 +82,7 @@ public:
     CharReach cr;
     CharReach pred_cr;
     CharReach succ_cr;
-    unsigned edge_top;
+    flat_set<u32> edge_tops; /**< tops on edge from start */
     unsigned equivalence_class;
     unsigned vertex_flags;
 };
@@ -120,7 +120,7 @@ public:
               EquivalenceType eq)
         : /* reports only matter for right-equiv */
           rs(eq == RIGHT_EQUIVALENCE ? g[vi.v].reports : flat_set<ReportID>()),
-          vertex_flags(vi.vertex_flags), edge_top(vi.edge_top), cr(vi.cr),
+          vertex_flags(vi.vertex_flags), edge_tops(vi.edge_tops), cr(vi.cr),
           adjacent_cr(eq == LEFT_EQUIVALENCE ? vi.pred_cr : vi.succ_cr),
           /* treat non-special vertices the same */
           node_type(min(g[vi.v].index, u32{N_SPECIALS})), depth(d_in) {}
@@ -128,7 +128,7 @@ public:
     bool operator==(const ClassInfo &b) const {
         return node_type == b.node_type && depth.d1 == b.depth.d1 &&
                depth.d2 == b.depth.d2 && cr == b.cr &&
-               adjacent_cr == b.adjacent_cr && edge_top == b.edge_top &&
+               adjacent_cr == b.adjacent_cr && edge_tops == b.edge_tops &&
                vertex_flags == b.vertex_flags && rs == b.rs;
     }
 
@@ -136,7 +136,6 @@ public:
         size_t val = 0;
         boost::hash_combine(val, boost::hash_range(begin(c.rs), end(c.rs)));
         boost::hash_combine(val, c.vertex_flags);
-        boost::hash_combine(val, c.edge_top);
         boost::hash_combine(val, c.cr);
         boost::hash_combine(val, c.adjacent_cr);
         boost::hash_combine(val, c.node_type);
@@ -148,7 +147,7 @@ public:
 private:
     flat_set<ReportID> rs; /* for right equiv only */
     unsigned vertex_flags;
-    u32 edge_top;
+    flat_set<u32> edge_tops;
     CharReach cr;
     CharReach adjacent_cr;
     unsigned node_type;
@@ -307,7 +306,7 @@ ptr_vector<VertexInfo> getVertexInfos(const NGHolder &g) {
 
             // also set up edge tops
             if (is_triggered(g) && u == g.start) {
-                cur_vi.edge_top = g[e].top;
+                cur_vi.edge_tops = g[e].tops;
             }
         }
 
@@ -544,7 +543,7 @@ void mergeClass(ptr_vector<VertexInfo> &infos, NGHolder &g, unsigned eq_class,
         infos.push_back(new_vertex_info_eod);
     }
 
-    const unsigned edgetop = (*cur_class_vertices.begin())->edge_top;
+    const auto &edgetops = (*cur_class_vertices.begin())->edge_tops;
     for (VertexInfo *old_vertex_info : cur_class_vertices) {
         assert(old_vertex_info->equivalence_class == eq_class);
 
@@ -565,9 +564,10 @@ void mergeClass(ptr_vector<VertexInfo> &infos, NGHolder &g, unsigned eq_class,
             // if edge doesn't exist, create it
             NFAEdge e = add_edge_if_not_present(pred_info->v, new_v, g).first;
 
-            // put edge top, if applicable
-            if (edgetop != (unsigned) -1) {
-                g[e].top = edgetop;
+            // put edge tops, if applicable
+            if (!edgetops.empty()) {
+                assert(g[e].tops.empty() || g[e].tops == edgetops);
+                g[e].tops = edgetops;
             }
 
             pred_info->succ.insert(new_vertex_info);
@@ -576,9 +576,10 @@ void mergeClass(ptr_vector<VertexInfo> &infos, NGHolder &g, unsigned eq_class,
                 NFAEdge ee = add_edge_if_not_present(pred_info->v, new_v_eod,
                                                      g).first;
 
-                // put edge top, if applicable
-                if (edgetop != (unsigned) -1) {
-                    g[ee].top = edgetop;
+                // put edge tops, if applicable
+                if (!edgetops.empty()) {
+                    assert(g[e].tops.empty() || g[e].tops == edgetops);
+                    g[ee].tops = edgetops;
                 }
 
                 pred_info->succ.insert(new_vertex_info_eod);
diff --git a/src/nfagraph/ng_graph.h b/src/nfagraph/ng_graph.h
index 64b32839..2d6fea13 100644
--- a/src/nfagraph/ng_graph.h
+++ b/src/nfagraph/ng_graph.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, Intel Corporation
+ * Copyright (c) 2015-2016, Intel Corporation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -69,8 +69,8 @@ struct NFAGraphEdgeProps {
     u32 index = 0;
 
     /** \brief For graphs that will be implemented as multi-top engines, this
-     * specifies the top event. Only used on edges from the start vertex. */
-    u32 top = 0;
+     * specifies the top events. Only used on edges from the start vertex. */
+    ue2::flat_set<u32> tops;
 
     /** \brief Flags associated with assertions. */
     u32 assert_flags = 0;
diff --git a/src/nfagraph/ng_holder.cpp b/src/nfagraph/ng_holder.cpp
index 53566891..5d83e626 100644
--- a/src/nfagraph/ng_holder.cpp
+++ b/src/nfagraph/ng_holder.cpp
@@ -178,7 +178,6 @@ std::pair<NFAEdge, bool> add_edge(NFAVertex u, NFAVertex v, NGHolder &h) {
     pair<NFAEdge, bool> e = add_edge(u, v, h.g);
     h.g[e.first].index = h.numEdges++;
     assert(!h.isValidNumEdges || h.numEdges > 0); // no wrapping
-    h.g[e.first].top = 0;
     return e;
 }
 
diff --git a/src/nfagraph/ng_holder.h b/src/nfagraph/ng_holder.h
index f0a387d0..49050808 100644
--- a/src/nfagraph/ng_holder.h
+++ b/src/nfagraph/ng_holder.h
@@ -315,6 +315,8 @@ void remove_edges(Iter begin, Iter end, NGHolder &h, bool renumber = true) {
     }
 }
 
+#define DEFAULT_TOP 0U
+
 /** \brief Clear and remove all of the edges pointed to by the edge descriptors
  * in the given container.
  *
diff --git a/src/nfagraph/ng_is_equal.cpp b/src/nfagraph/ng_is_equal.cpp
index cc65fa17..8e71c337 100644
--- a/src/nfagraph/ng_is_equal.cpp
+++ b/src/nfagraph/ng_is_equal.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, Intel Corporation
+ * Copyright (c) 2015-2016, Intel Corporation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -153,14 +153,14 @@ bool is_equal_i(const NGHolder &a, const NGHolder &b,
     }
 
     /* check top for edges out of start */
-    vector<pair<u32, u32>> top_a;
-    vector<pair<u32, u32>> top_b;
+    vector<pair<u32, flat_set<u32>>> top_a;
+    vector<pair<u32, flat_set<u32>>> top_b;
 
     for (const auto &e : out_edges_range(a.start, a)) {
-        top_a.emplace_back(a[target(e, a)].index, a[e].top);
+        top_a.emplace_back(a[target(e, a)].index, a[e].tops);
     }
     for (const auto &e : out_edges_range(b.start, b)) {
-        top_b.emplace_back(b[target(e, b)].index, b[e].top);
+        top_b.emplace_back(b[target(e, b)].index, b[e].tops);
     }
 
     sort(top_a.begin(), top_a.end());
diff --git a/src/nfagraph/ng_limex.cpp b/src/nfagraph/ng_limex.cpp
index 06ea5de3..b9f3434b 100644
--- a/src/nfagraph/ng_limex.cpp
+++ b/src/nfagraph/ng_limex.cpp
@@ -148,14 +148,16 @@ void dropRedundantStartEdges(NGHolder &g) {
 static
 void makeTopStates(NGHolder &g, map<u32, NFAVertex> &tops,
                    const map<u32, CharReach> &top_reach) {
+    /* TODO: more intelligent creation of top states */
     map<u32, vector<NFAVertex>> top_succs;
     for (const auto &e : out_edges_range(g.start, g)) {
         NFAVertex v = target(e, g);
         if (v == g.startDs) {
             continue;
         }
-        u32 t = g[e].top;
-        top_succs[t].push_back(v);
+        for (u32 t : g[e].tops) {
+            top_succs[t].push_back(v);
+        }
     }
 
     for (const auto &top : top_succs) {
diff --git a/src/nfagraph/ng_redundancy.cpp b/src/nfagraph/ng_redundancy.cpp
index 26599251..8fc5d5f3 100644
--- a/src/nfagraph/ng_redundancy.cpp
+++ b/src/nfagraph/ng_redundancy.cpp
@@ -310,7 +310,7 @@ bool hasInEdgeTops(const NGHolder &g, NFAVertex v) {
     bool exists;
     NFAEdge e;
     tie(e, exists) = edge_by_target(g.start, v, g);
-    if (exists && g[e].top != 0) {
+    if (exists && !g[e].tops.empty()) {
         return true;
     }
     return false;
diff --git a/src/nfagraph/ng_repeat.cpp b/src/nfagraph/ng_repeat.cpp
index bc7e73d3..5bff21b0 100644
--- a/src/nfagraph/ng_repeat.cpp
+++ b/src/nfagraph/ng_repeat.cpp
@@ -215,8 +215,8 @@ bool rogueSuccessor(const NGHolder &g, NFAVertex v,
 
 static
 bool hasDifferentTops(const NGHolder &g, const vector<NFAVertex> &verts) {
-    bool found = false;
-    u32 top = 0;
+    /* TODO: check that we need this now that we allow multiple tops */
+    const flat_set<u32> *tops = nullptr;
 
     for (auto v : verts) {
         for (const auto &e : in_edges_range(v, g)) {
@@ -224,17 +224,12 @@ bool hasDifferentTops(const NGHolder &g, const vector<NFAVertex> &verts) {
             if (u != g.start && u != g.startDs) {
                 continue; // Only edges from starts have valid top properties.
             }
-            u32 t = g[e].top;
-            DEBUG_PRINTF("edge (%u,%u) with top %u\n", g[u].index,
-                         g[v].index, t);
-            assert(t < NFA_MAX_TOP_MASKS);
-            if (!found) {
-                found = true;
-                top = t;
-            } else {
-                if (t != top) {
-                    return true; // More than one top.
-                }
+            DEBUG_PRINTF("edge (%u,%u) with %zu tops\n", g[u].index, g[v].index,
+                         g[e].tops.size());
+            if (!tops) {
+                tops = &g[e].tops;
+            } else if (g[e].tops != *tops) {
+                return true; // More than one set of tops.
             }
         }
     }
@@ -1123,7 +1118,7 @@ NFAVertex buildTriggerStates(NGHolder &g, const vector<CharReach> &trigger,
         g[v].char_reach = cr;
         add_edge(u, v, g);
         if (u == g.start) {
-            g[edge(u, v, g).first].top = top;
+            g[edge(u, v, g).first].tops.insert(top);
         }
         u = v;
     }
@@ -1153,18 +1148,21 @@ void addTriggers(NGHolder &g,
             continue;
         }
 
-        const auto &top = g[e].top;
+        const auto &tops = g[e].tops;
 
         // The caller may not have given us complete trigger information. If we
         // don't have any triggers for a particular top, we should just leave
         // it alone.
-        if (!contains(triggers, top)) {
-            DEBUG_PRINTF("no triggers for top %u\n", top);
-            continue;
-        }
+        for (u32 top : tops) {
+            if (!contains(triggers, top)) {
+                DEBUG_PRINTF("no triggers for top %u\n", top);
+                goto next_edge;
+            }
 
-        starts_by_top[top].push_back(v);
+            starts_by_top[top].push_back(v);
+        }
         dead.push_back(e);
+    next_edge:;
     }
 
     remove_edges(dead, g);
@@ -2105,14 +2103,26 @@ void populateFixedTopInfo(const map<u32, u32> &fixed_depth_tops,
         if (v == g.startDs) {
             continue;
         }
-        u32 top = g[e].top;
+
         depth td = depth::infinity();
-        if (contains(fixed_depth_tops, top)) {
-            td = fixed_depth_tops.at(top);
+        for (u32 top : g[e].tops) {
+            if (!contains(fixed_depth_tops, top)) {
+                td = depth::infinity();
+                break;
+            }
+            depth td_t = fixed_depth_tops.at(top);
+            if (td == td_t) {
+                continue;
+            } else if (td == depth::infinity()) {
+                td = td_t;
+            } else {
+                td = depth::infinity();
+                break;
+            }
         }
 
-        DEBUG_PRINTF("scanning from %u top=%u depth=%s\n",
-                     g[v].index, top, td.str().c_str());
+        DEBUG_PRINTF("scanning from %u depth=%s\n", g[v].index,
+                     td.str().c_str());
         /* for each vertex reachable from v update its map to reflect that it is
          * reachable from a top of depth td. */
 
@@ -2428,7 +2438,7 @@ bool isPureRepeat(const NGHolder &g, PureRepeat &repeat) {
     }
 
     // Must have precisely one top.
-    if (!onlyOneTop(g)) {
+    if (is_triggered(g) && !onlyOneTop(g)) {
         DEBUG_PRINTF("Too many tops\n");
         return false;
     }
diff --git a/src/nfagraph/ng_restructuring.cpp b/src/nfagraph/ng_restructuring.cpp
index 09abf775..c85860c7 100644
--- a/src/nfagraph/ng_restructuring.cpp
+++ b/src/nfagraph/ng_restructuring.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, Intel Corporation
+ * Copyright (c) 2015-2016, Intel Corporation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -174,6 +174,7 @@ numberStates(NGHolder &h, const map<u32, NFAVertex> &tops) {
 u32 countStates(const NGHolder &g,
                 const ue2::unordered_map<NFAVertex, u32> &state_ids,
                 bool addTops) {
+    /* TODO: smarter top state allocation, move to limex? */
     if (state_ids.empty()) {
         return 0;
     }
@@ -188,11 +189,11 @@ u32 countStates(const NGHolder &g,
     u32 num_states = max_state + 1;
 
     assert(contains(state_ids, g.start));
-    if (addTops && state_ids.at(g.start) != NO_STATE) {
+    if (addTops && is_triggered(g) && state_ids.at(g.start) != NO_STATE) {
         num_states--;
         set<u32> tops;
         for (auto e : out_edges_range(g.start, g)) {
-            tops.insert(g[e].top);
+            insert(&tops, g[e].tops);
         }
         num_states += tops.size();
     }
diff --git a/src/nfagraph/ng_rose.cpp b/src/nfagraph/ng_rose.cpp
index 137ac5cc..108134a6 100644
--- a/src/nfagraph/ng_rose.cpp
+++ b/src/nfagraph/ng_rose.cpp
@@ -811,6 +811,7 @@ bool can_match(const NGHolder &g, const ue2_literal &lit, bool overhang_ok) {
 
 u32 removeTrailingLiteralStates(NGHolder &g, const ue2_literal &lit,
                                 u32 max_delay, bool overhang_ok) {
+    assert(isCorrectlyTopped(g));
     if (max_delay == MO_INVALID_IDX) {
         max_delay--;
     }
@@ -878,12 +879,16 @@ u32 removeTrailingLiteralStates(NGHolder &g, const ue2_literal &lit,
     sort(verts.begin(), verts.end(), VertexIndexOrdering<NGHolder>(g));
 
     for (auto v : verts) {
-        add_edge(v, g.accept, g);
+        NFAEdge e = add_edge(v, g.accept, g).first;
         g[v].reports.insert(0);
+        if (is_triggered(g) && v == g.start) {
+            g[e].tops.insert(DEFAULT_TOP);
+        }
     }
 
     pruneUseless(g);
     assert(allMatchStatesHaveReports(g));
+    assert(isCorrectlyTopped(g));
 
     DEBUG_PRINTF("graph has %zu vertices left\n", num_vertices(g));
     return delay;
@@ -892,6 +897,7 @@ u32 removeTrailingLiteralStates(NGHolder &g, const ue2_literal &lit,
 void restoreTrailingLiteralStates(NGHolder &g, const ue2_literal &lit,
                                   u32 delay, const vector<NFAVertex> &preds) {
     assert(delay <= lit.length());
+    assert(isCorrectlyTopped(g));
     DEBUG_PRINTF("adding on '%s' %u\n", dumpString(lit).c_str(), delay);
 
     NFAVertex prev = g.accept;
@@ -906,7 +912,10 @@ void restoreTrailingLiteralStates(NGHolder &g, const ue2_literal &lit,
     }
 
     for (auto v : preds) {
-        add_edge(v, prev, g);
+        NFAEdge e = add_edge(v, prev, g).first;
+        if (v == g.start && is_triggered(g)) {
+            g[e].tops.insert(DEFAULT_TOP);
+        }
     }
 
     // Every predecessor of accept must have a report.
@@ -917,6 +926,7 @@ void restoreTrailingLiteralStates(NGHolder &g, const ue2_literal &lit,
     g.renumberVertices();
     g.renumberEdges();
     assert(allMatchStatesHaveReports(g));
+    assert(isCorrectlyTopped(g));
 }
 
 void restoreTrailingLiteralStates(NGHolder &g, const ue2_literal &lit,
diff --git a/src/nfagraph/ng_split.cpp b/src/nfagraph/ng_split.cpp
index bce638c0..4576a498 100644
--- a/src/nfagraph/ng_split.cpp
+++ b/src/nfagraph/ng_split.cpp
@@ -151,7 +151,8 @@ void splitRHS(const NGHolder &base, const vector<NFAVertex> &pivots,
 
     for (auto pivot : pivots) {
         assert(contains(*rhs_map, pivot));
-        add_edge(rhs->start, (*rhs_map)[pivot], *rhs);
+        NFAEdge e = add_edge(rhs->start, (*rhs_map)[pivot], *rhs).first;
+        (*rhs)[e].tops.insert(DEFAULT_TOP);
     }
 
      /* should do the renumbering unconditionally as we know edges are already
@@ -215,6 +216,7 @@ void splitGraph(const NGHolder &base, const vector<NFAVertex> &pivots,
     DEBUG_PRINTF("splitting graph at %zu vertices\n", pivots.size());
 
     assert(!has_parallel_edge(base));
+    assert(isCorrectlyTopped(base));
 
     /* RHS pivots are built from the common set of successors of pivots. */
     vector<NFAVertex> rhs_pivots;
@@ -228,6 +230,8 @@ void splitGraph(const NGHolder &base, const vector<NFAVertex> &pivots,
 
     assert(!has_parallel_edge(*lhs));
     assert(!has_parallel_edge(*rhs));
+    assert(isCorrectlyTopped(*lhs));
+    assert(isCorrectlyTopped(*rhs));
 }
 
 void splitGraph(const NGHolder &base, NFAVertex pivot,
diff --git a/src/nfagraph/ng_split.h b/src/nfagraph/ng_split.h
index 75577e97..31c1cf35 100644
--- a/src/nfagraph/ng_split.h
+++ b/src/nfagraph/ng_split.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, Intel Corporation
+ * Copyright (c) 2015-2016, Intel Corporation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -47,6 +47,8 @@ class NGHolder;
  * is in the lhs if it is reachable from start without going through the
  * pivot. The pivot ends up in the LHS and any adjacent vertices in the RHS.
  *
+ * Note: The RHS is setup to be triggered by TOP 0
+ *
  * When multiple split vertices are provided:
  * - RHS contains all vertices reachable from every pivot
  * - LHS contains all vertices which are reachable from start ignoring any
diff --git a/src/nfagraph/ng_uncalc_components.cpp b/src/nfagraph/ng_uncalc_components.cpp
index 217183de..fd6dfc3e 100644
--- a/src/nfagraph/ng_uncalc_components.cpp
+++ b/src/nfagraph/ng_uncalc_components.cpp
@@ -205,11 +205,10 @@ u32 commonPrefixLength(const NGHolder &ga,
                     break;
                 }
 
-                if (ga[*ei].top != gb[b_edge].top) {
+                if (ga[*ei].tops != gb[b_edge].tops) {
                     max = i;
                     ok = false;
-                    DEBUG_PRINTF("tops don't match on edge %zu->%u\n",
-                                 i, sid);
+                    DEBUG_PRINTF("tops don't match on edge %zu->%u\n", i, sid);
                 }
             }
 
@@ -318,7 +317,7 @@ void mergeNfa(NGHolder &dest, vector<NFAVertex> &destStateMap,
                 DEBUG_PRINTF("skipping common edge\n");
                 assert(edge(u, v, dest).second);
                 // Should never merge edges with different top values.
-                assert(vic[e].top == dest[edge(u, v, dest).first].top);
+                assert(vic[e].tops == dest[edge(u, v, dest).first].tops);
                 continue;
             } else {
                 assert(is_any_accept(v, dest));
@@ -506,11 +505,13 @@ bool mergeableStarts(const NGHolder &h1, const NGHolder &h2) {
         return false;
     }
 
+    /* TODO: relax top checks if reports match */
+
     // If both graphs have edge (start, accept), the tops must match.
     auto e1_accept = edge(h1.start, h1.accept, h1);
     auto e2_accept = edge(h2.start, h2.accept, h2);
     if (e1_accept.second && e2_accept.second &&
-        h1[e1_accept.first].top != h2[e2_accept.first].top) {
+        h1[e1_accept.first].tops != h2[e2_accept.first].tops) {
         return false;
     }
 
@@ -518,7 +519,7 @@ bool mergeableStarts(const NGHolder &h1, const NGHolder &h2) {
     auto e1_eod = edge(h1.start, h1.acceptEod, h1);
     auto e2_eod = edge(h2.start, h2.acceptEod, h2);
     if (e1_eod.second && e2_eod.second &&
-        h1[e1_eod.first].top != h2[e2_eod.first].top) {
+        h1[e1_eod.first].tops != h2[e2_eod.first].tops) {
         return false;
     }
 
diff --git a/src/nfagraph/ng_util.cpp b/src/nfagraph/ng_util.cpp
index c629d553..da9c2438 100644
--- a/src/nfagraph/ng_util.cpp
+++ b/src/nfagraph/ng_util.cpp
@@ -165,12 +165,7 @@ void clone_in_edges(NGHolder &g, NFAVertex s, NFAVertex dest) {
 }
 
 bool onlyOneTop(const NGHolder &g) {
-    set<u32> tops;
-    for (const auto &e : out_edges_range(g.start, g)) {
-        tops.insert(g[e].top);
-    }
-    assert(!tops.empty());
-    return tops.size() == 1;
+    return getTops(g).size() == 1;
 }
 
 namespace {
@@ -465,17 +460,21 @@ void appendLiteral(NGHolder &h, const ue2_literal &s) {
 ue2::flat_set<u32> getTops(const NGHolder &h) {
     ue2::flat_set<u32> tops;
     for (const auto &e : out_edges_range(h.start, h)) {
-        NFAVertex v = target(e, h);
-        if (v == h.startDs) {
-            continue;
-        }
-        u32 top = h[e].top;
-        assert(top < NFA_MAX_TOP_MASKS);
-        tops.insert(top);
+        insert(&tops, h[e].tops);
     }
     return tops;
 }
 
+void setTops(NGHolder &h, u32 top) {
+    for (const auto &e : out_edges_range(h.start, h)) {
+        assert(h[e].tops.empty());
+        if (target(e, h) == h.startDs) {
+            continue;
+        }
+        h[e].tops.insert(top);
+    }
+}
+
 void clearReports(NGHolder &g) {
     DEBUG_PRINTF("clearing reports without an accept edge\n");
     ue2::unordered_set<NFAVertex> allow;
@@ -694,6 +693,25 @@ bool hasCorrectlyNumberedEdges(const NGHolder &g) {
         && num_edges(g) == num_edges(g.g);
 }
 
+bool isCorrectlyTopped(const NGHolder &g) {
+    if (is_triggered(g)) {
+        for (const auto &e : out_edges_range(g.start, g)) {
+            if (g[e].tops.empty() != (target(e, g) == g.startDs)) {
+                return false;
+            }
+        }
+    } else {
+        for (const auto &e : out_edges_range(g.start, g)) {
+            if (!g[e].tops.empty()) {
+                return false;
+            }
+        }
+    }
+
+    return true;
+}
+
+
 #endif // NDEBUG
 
 } // namespace ue2
diff --git a/src/nfagraph/ng_util.h b/src/nfagraph/ng_util.h
index 4f58dc45..1c6dd461 100644
--- a/src/nfagraph/ng_util.h
+++ b/src/nfagraph/ng_util.h
@@ -198,9 +198,13 @@ VertexIndexOrdering<Graph> make_index_ordering(const Graph &g) {
 
 bool onlyOneTop(const NGHolder &g);
 
-/** Return a mask of the tops on the given graph. */
+/** Return the set of the tops on the given graph. */
 flat_set<u32> getTops(const NGHolder &h);
 
+/** Initialise the tops on h to the provide top. Assumes that h is triggered and
+ * no tops have been set on h. */
+void setTops(NGHolder &h, u32 top = DEFAULT_TOP);
+
 /** adds a vertex to g with all the same vertex properties as \p v (aside from
  * index) */
 NFAVertex clone_vertex(NGHolder &g, NFAVertex v);
@@ -319,6 +323,12 @@ bool hasCorrectlyNumberedVertices(const NGHolder &g);
  */
 bool hasCorrectlyNumberedEdges(const NGHolder &g);
 
+/**
+ * Assertion: returns true if the graph is triggered and all edges out of start
+ * have tops OR if the graph is not-triggered and all edges out of start have no
+ * tops.
+ */
+bool isCorrectlyTopped(const NGHolder &g);
 #endif // NDEBUG
 
 } // namespace ue2
diff --git a/src/nfagraph/ng_violet.cpp b/src/nfagraph/ng_violet.cpp
index 94e0a998..538c945d 100644
--- a/src/nfagraph/ng_violet.cpp
+++ b/src/nfagraph/ng_violet.cpp
@@ -1076,8 +1076,10 @@ bool splitRoseEdge(const NGHolder &base_graph, RoseInGraph &vg,
 
     assert(hasCorrectlyNumberedVertices(*rhs));
     assert(hasCorrectlyNumberedEdges(*rhs));
+    assert(isCorrectlyTopped(*rhs));
     assert(hasCorrectlyNumberedVertices(*lhs));
     assert(hasCorrectlyNumberedEdges(*lhs));
+    assert(isCorrectlyTopped(*lhs));
 
     return true;
 }
@@ -1152,7 +1154,11 @@ void splitEdgesByCut(NGHolder &h, RoseInGraph &vg,
             /* want to cut off paths to pivot from things other than the pivot -
              * makes a more svelte graphy */
             clear_in_edges(temp_map[pivot], *new_lhs);
-            add_edge(temp_map[prev_v], temp_map[pivot], *new_lhs);
+            NFAEdge pivot_edge = add_edge(temp_map[prev_v], temp_map[pivot],
+                                          *new_lhs).first;
+            if (is_triggered(h) && prev_v == h.start) {
+                (*new_lhs)[pivot_edge].tops.insert(DEFAULT_TOP);
+            }
 
             pruneUseless(*new_lhs, false);
             renumber_vertices(*new_lhs);
@@ -1162,6 +1168,7 @@ void splitEdgesByCut(NGHolder &h, RoseInGraph &vg,
 
             assert(hasCorrectlyNumberedVertices(*new_lhs));
             assert(hasCorrectlyNumberedEdges(*new_lhs));
+            assert(isCorrectlyTopped(*new_lhs));
 
             const set<ue2_literal> &lits = cut_lits.at(e);
             for (const auto &lit : lits) {
@@ -1228,6 +1235,7 @@ void splitEdgesByCut(NGHolder &h, RoseInGraph &vg,
                 DEBUG_PRINTF("    into rhs %s\n",
                               to_string(new_rhs->kind).c_str());
                 done_rhs.emplace(adj, new_rhs);
+                assert(isCorrectlyTopped(*new_rhs));
             }
 
             assert(done_rhs[adj].get());
@@ -1235,6 +1243,7 @@ void splitEdgesByCut(NGHolder &h, RoseInGraph &vg,
 
             assert(hasCorrectlyNumberedVertices(*new_rhs));
             assert(hasCorrectlyNumberedEdges(*new_rhs));
+            assert(isCorrectlyTopped(*new_rhs));
 
             if (vg[dest].type == RIV_LITERAL
                 && !can_match(*new_rhs, vg[dest].s, true)) {
@@ -1380,6 +1389,7 @@ void avoidOutfixes(RoseInGraph &vg, const CompileContext &cc) {
     RoseInEdge e = *edges(vg).first;
 
     NGHolder &h = *vg[e].graph;
+    assert(isCorrectlyTopped(h));
 
     renumber_vertices(h);
     renumber_edges(h);
@@ -1602,6 +1612,7 @@ void removeRedundantLiteralsFromInfix(const NGHolder &h, RoseInGraph &ig,
             continue;
         }
 
+        assert(isCorrectlyTopped(*h_new));
         graphs[right] = make_pair(h_new, delay);
     }
 
@@ -1720,6 +1731,8 @@ unique_ptr<NGHolder> make_chain(u32 count) {
     h[u].reports.insert(0);
     add_edge(u, h.accept, h);
 
+    setTops(h);
+
     return rv;
 }
 
@@ -1777,6 +1790,7 @@ bool makeTransientFromLongLiteral(NGHolder &h, RoseInGraph &vg,
         assert(willBeTransient(findMaxWidth(*h_new), cc)
                || willBeAnchoredTable(findMaxWidth(*h_new), cc.grey));
 
+        assert(isCorrectlyTopped(*h_new));
         graphs[v] = h_new;
     }
 
@@ -1811,6 +1825,7 @@ bool improvePrefix(NGHolder &h, RoseInGraph &vg, const vector<RoseInEdge> &ee,
                    const CompileContext &cc) {
     DEBUG_PRINTF("trying to improve prefix %p, %zu verts\n", &h,
                   num_vertices(h));
+    assert(isCorrectlyTopped(h));
 
     renumber_vertices(h);
     renumber_edges(h);
@@ -1860,6 +1875,7 @@ bool improvePrefix(NGHolder &h, RoseInGraph &vg, const vector<RoseInEdge> &ee,
         for (const auto &e : ee) {
             shared_ptr<NGHolder> hh = cloneHolder(h);
             auto succ_lit = vg[target(e, vg)].s;
+            assert(isCorrectlyTopped(*hh));
             u32 delay = removeTrailingLiteralStates(*hh, succ_lit,
                                                     succ_lit.length(),
                                               false /* can't overhang start */);
@@ -1868,6 +1884,7 @@ bool improvePrefix(NGHolder &h, RoseInGraph &vg, const vector<RoseInEdge> &ee,
                 continue;
             }
 
+            assert(isCorrectlyTopped(*hh));
             trimmed[hh].emplace_back(e, delay);
         }
 
@@ -2110,10 +2127,15 @@ void splitEdgesForSuffix(const NGHolder &base_graph, RoseInGraph &vg,
     add_edge(lhs->accept, lhs->acceptEod, *lhs);
     clearReports(*lhs);
     for (NFAVertex v : splitters) {
-        add_edge(v_map[v], lhs->accept, *lhs);
+        NFAEdge e = add_edge(v_map[v], lhs->accept, *lhs).first;
+        if (v == base_graph.start) {
+            (*lhs)[e].tops.insert(DEFAULT_TOP);
+        }
         (*lhs)[v_map[v]].reports.insert(0);
+
     }
     pruneUseless(*lhs);
+    assert(isCorrectlyTopped(*lhs));
 
     /* create literal vertices and connect preds */
     for (const auto &lit : split.lit) {
diff --git a/src/nfagraph/ng_width.cpp b/src/nfagraph/ng_width.cpp
index 470f9343..5fb58ee4 100644
--- a/src/nfagraph/ng_width.cpp
+++ b/src/nfagraph/ng_width.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, Intel Corporation
+ * Copyright (c) 2015-2016, Intel Corporation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -69,7 +69,7 @@ struct SpecialEdgeFilter {
             return false;
         }
         if (single_top) {
-            if (u == h->start && g[e].top != top) {
+            if (u == h->start && !contains(g[e].tops, top)) {
                 return false;
             }
             if (u == h->startDs) {
diff --git a/src/rose/rose_build_add.cpp b/src/rose/rose_build_add.cpp
index 0f0e8d18..72a791ba 100644
--- a/src/rose/rose_build_add.cpp
+++ b/src/rose/rose_build_add.cpp
@@ -1619,6 +1619,8 @@ bool RoseBuildImpl::addRose(const RoseInGraph &ig, bool prefilter,
         }
 
         NGHolder *h = in[e].graph.get();
+
+        assert(isCorrectlyTopped(*h));
         if (!contains(graphs, h)) {
             ordered_graphs.push_back(h);
         }
diff --git a/src/rose/rose_build_add_mask.cpp b/src/rose/rose_build_add_mask.cpp
index 45333a38..ef83cae1 100644
--- a/src/rose/rose_build_add_mask.cpp
+++ b/src/rose/rose_build_add_mask.cpp
@@ -574,7 +574,8 @@ unique_ptr<NGHolder> buildMaskRhs(const ue2::flat_set<ReportID> &reports,
         succ = u;
     }
 
-    add_edge(h.start, succ, h);
+    NFAEdge e = add_edge(h.start, succ, h).first;
+    h[e].tops.insert(DEFAULT_TOP);
 
     return rhs;
 }
@@ -632,6 +633,7 @@ void doAddMask(RoseBuildImpl &tbi, bool anchored,
                     = buildMaskLhs(true, minBound - prefix2_len + overlap,
                                    mask3);
                 mhs->kind = NFA_INFIX;
+                setTops(*mhs);
                 add_edge(u, v, RoseInEdgeProps(mhs, delay), ig);
 
                 DEBUG_PRINTF("add anch literal too!\n");
diff --git a/src/rose/rose_build_bytecode.cpp b/src/rose/rose_build_bytecode.cpp
index 5421f1cb..a7979c4f 100644
--- a/src/rose/rose_build_bytecode.cpp
+++ b/src/rose/rose_build_bytecode.cpp
@@ -512,7 +512,7 @@ bool nfaStuckOn(const NGHolder &g) {
     set<u32> done_tops;
 
     for (const auto &e : out_edges_range(g.start, g)) {
-        tops.insert(g[e].top);
+        insert(&tops, g[e].tops);
         if (!g[target(e, g)].char_reach.all()) {
             continue;
         }
@@ -521,7 +521,7 @@ bool nfaStuckOn(const NGHolder &g) {
         insert(&asucc, adjacent_vertices(target(e, g), g));
 
         if (asucc == succ) {
-            done_tops.insert(g[e].top);
+            insert(&done_tops, g[e].tops);
         }
     }
 
@@ -842,8 +842,8 @@ makeLeftNfa(const RoseBuildImpl &tbi, left_id &left,
     if (!n && !is_prefix && left.graph() && onlyOneTop(*left.graph())) {
         map<u32, vector<vector<CharReach> > > triggers;
         findTriggerSequences(tbi, infixTriggers.at(left), &triggers);
-        assert(contains(triggers, 0)); // single top
-        n = constructLBR(*left.graph(), triggers[0], cc, rm);
+        assert(triggers.size() == 1); // single top
+        n = constructLBR(*left.graph(), triggers.begin()->second, cc, rm);
     }
 
     if (!n && left.graph()) {
@@ -1435,7 +1435,7 @@ void findExclusiveInfixes(RoseBuildImpl &build, build_context &bc,
 
         // Sanity check: our NFA should contain each of the tops mentioned on
         // our in-edges.
-        assert(roseHasTops(g, v));
+        assert(roseHasTops(build, v));
 
         if (contains(leftfixes, leftfix)) {
             // NFA already built.
@@ -1504,7 +1504,7 @@ bool buildLeftfixes(RoseBuildImpl &tbi, build_context &bc,
 
         // Sanity check: our NFA should contain each of the tops mentioned on
         // our in-edges.
-        assert(roseHasTops(g, v));
+        assert(roseHasTops(tbi, v));
 
         bool is_transient = contains(tbi.transient, leftfix);
 
diff --git a/src/rose/rose_build_compile.cpp b/src/rose/rose_build_compile.cpp
index 3f82a9cc..c93f4eac 100644
--- a/src/rose/rose_build_compile.cpp
+++ b/src/rose/rose_build_compile.cpp
@@ -43,7 +43,6 @@
 #include "nfa/nfa_internal.h"
 #include "nfa/rdfa.h"
 #include "nfagraph/ng_holder.h"
-#include "nfagraph/ng_dump.h"
 #include "nfagraph/ng_execute.h"
 #include "nfagraph/ng_is_equal.h"
 #include "nfagraph/ng_limex.h"
@@ -1554,53 +1553,6 @@ bool roleOffsetsAreValid(const RoseGraph &g) {
     }
     return true;
 }
-
-static UNUSED
-bool hasOrphanedTops(const RoseBuildImpl &tbi) {
-    const RoseGraph &g = tbi.g;
-
-    ue2::unordered_map<left_id, set<u32> > roses;
-    ue2::unordered_map<suffix_id, set<u32> > suffixes;
-
-    for (auto v : vertices_range(g)) {
-        if (g[v].left) {
-            set<u32> &tops = roses[g[v].left];
-            if (tbi.isRootSuccessor(v)) {
-                // Prefix, has only one top.
-                tops.insert(0);
-            } else {
-                // Tops for infixes come from the in-edges.
-                for (const auto &e : in_edges_range(v, g)) {
-                    tops.insert(g[e].rose_top);
-                }
-            }
-        }
-        if (g[v].suffix) {
-            suffixes[g[v].suffix].insert(g[v].suffix.top);
-        }
-    }
-
-    for (const auto &e : roses) {
-        if (all_tops(e.first) != e.second) {
-            DEBUG_PRINTF("rose tops (%s) don't match rose graph (%s)\n",
-                         as_string_list(all_tops(e.first)).c_str(),
-                         as_string_list(e.second).c_str());
-            return true;
-        }
-    }
-
-    for (const auto &e : suffixes) {
-        if (all_tops(e.first) != e.second) {
-            DEBUG_PRINTF("suffix tops (%s) don't match rose graph (%s)\n",
-                         as_string_list(all_tops(e.first)).c_str(),
-                         as_string_list(e.second).c_str());
-            return true;
-        }
-    }
-
-    return false;
-}
-
 #endif // NDEBUG
 
 aligned_unique_ptr<RoseEngine> RoseBuildImpl::buildRose(u32 minWidth) {
@@ -1681,13 +1633,17 @@ aligned_unique_ptr<RoseEngine> RoseBuildImpl::buildRose(u32 minWidth) {
         mergeSmallLeftfixes(*this);
     }
 
+    assert(!hasOrphanedTops(*this));
+
     // Do a rose-merging aliasing pass.
     aliasRoles(*this, true);
+    assert(!hasOrphanedTops(*this));
 
     // Run a merge pass over the outfixes as well.
     mergeOutfixes(*this);
 
     assert(!danglingVertexRef(*this));
+    assert(!hasOrphanedTops(*this));
 
     findMoreLiteralMasks(*this);
 
diff --git a/src/rose/rose_build_convert.cpp b/src/rose/rose_build_convert.cpp
index 1578dda1..d3fa1ac6 100644
--- a/src/rose/rose_build_convert.cpp
+++ b/src/rose/rose_build_convert.cpp
@@ -163,6 +163,8 @@ unique_ptr<NGHolder> convertLeafToHolder(const RoseGraph &g,
         }
     }
 
+    setTops(*out);
+
     // Literal vertices wired to accept.
     NFAVertex litfirst, litlast;
     tie(litfirst, litlast) = addLiteralVertices(g, literals, t_v, *out);
@@ -400,7 +402,10 @@ unique_ptr<NGHolder> makeFloodProneSuffix(const ue2_literal &s, size_t len,
     NFAVertex u = h->start;
     for (auto it = s.begin() + s.length() - len; it != s.end(); ++it) {
         NFAVertex v = addHolderVertex(*it, *h);
-        add_edge(u, v, *h);
+        NFAEdge e = add_edge(u, v, *h).first;
+        if (u == h->start) {
+            (*h)[e].tops.insert(DEFAULT_TOP);
+        }
         u = v;
     }
 
diff --git a/src/rose/rose_build_impl.h b/src/rose/rose_build_impl.h
index d239a698..cc00603a 100644
--- a/src/rose/rose_build_impl.h
+++ b/src/rose/rose_build_impl.h
@@ -615,7 +615,8 @@ ue2_literal findNonOverlappingTail(const std::set<ue2_literal> &lits,
 void setReportId(NGHolder &g, ReportID id);
 
 #ifndef NDEBUG
-bool roseHasTops(const RoseGraph &g, RoseVertex v);
+bool roseHasTops(const RoseBuildImpl &build, RoseVertex v);
+bool hasOrphanedTops(const RoseBuildImpl &build);
 #endif
 
 u64a findMaxOffset(const std::set<ReportID> &reports, const ReportManager &rm);
diff --git a/src/rose/rose_build_infix.cpp b/src/rose/rose_build_infix.cpp
index e81a7b00..73f9e99b 100644
--- a/src/rose/rose_build_infix.cpp
+++ b/src/rose/rose_build_infix.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, Intel Corporation
+ * Copyright (c) 2015-2016, Intel Corporation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -108,15 +108,10 @@ void contractVertex(NGHolder &g, NFAVertex v,
 }
 
 static
-u32 findMaxInfixMatches(const NGHolder &h, const set<ue2_literal> &lits) {
+u32 findMaxLiteralMatches(const NGHolder &h, const set<ue2_literal> &lits) {
     DEBUG_PRINTF("h=%p, %zu literals\n", &h, lits.size());
     //dumpGraph("infix.dot", h.g);
 
-    if (!onlyOneTop(h)) {
-        DEBUG_PRINTF("more than one top!n");
-        return NO_MATCH_LIMIT;
-    }
-
     // Indices of vertices that could terminate any of the literals in 'lits'.
     set<u32> terms;
 
@@ -262,7 +257,11 @@ u32 findMaxInfixMatches(const left_id &left, const set<ue2_literal> &lits) {
         return findMaxInfixMatches(*left.castle(), lits);
     }
     if (left.graph()) {
-        return findMaxInfixMatches(*left.graph(), lits);
+        if (!onlyOneTop(*left.graph())) {
+            DEBUG_PRINTF("more than one top!n");
+            return NO_MATCH_LIMIT;
+        }
+        return findMaxLiteralMatches(*left.graph(), lits);
     }
 
     return NO_MATCH_LIMIT;
@@ -315,7 +314,7 @@ void findCountingMiracleInfo(const left_id &left, const vector<u8> &stopTable,
         lits.insert(ue2_literal(c, false));
     }
 
-    u32 count = findMaxInfixMatches(*left.graph(), lits);
+    u32 count = findMaxLiteralMatches(*left.graph(), lits);
     DEBUG_PRINTF("counting miracle %u\n", count + 1);
     if (count && count < 50) {
         *cm_count = count + 1;
diff --git a/src/rose/rose_build_lookaround.cpp b/src/rose/rose_build_lookaround.cpp
index ba77b402..7c58f931 100644
--- a/src/rose/rose_build_lookaround.cpp
+++ b/src/rose/rose_build_lookaround.cpp
@@ -72,7 +72,7 @@ void getForwardReach(const NGHolder &g, u32 top, map<s32, CharReach> &look) {
         if (v == g.startDs) {
             continue;
         }
-        if (g[e].top == top) {
+        if (contains(g[e].tops, top)) {
             curr.insert(v);
         }
     }
diff --git a/src/rose/rose_build_merge.cpp b/src/rose/rose_build_merge.cpp
index dbd580ed..01134736 100644
--- a/src/rose/rose_build_merge.cpp
+++ b/src/rose/rose_build_merge.cpp
@@ -1762,9 +1762,12 @@ void replaceTops(NGHolder &h, const map<u32, u32> &top_mapping) {
         if (v == h.startDs) {
             continue;
         }
-        DEBUG_PRINTF("vertex %u has top %u\n", h[v].index, h[e].top);
-        assert(contains(top_mapping, h[e].top));
-        h[e].top = top_mapping.at(h[e].top);
+        flat_set<u32> new_tops;
+        for (u32 t : h[e].tops) {
+            DEBUG_PRINTF("vertex %u has top %u\n", h[v].index, t);
+            new_tops.insert(top_mapping.at(t));
+        }
+        h[e].tops = move(new_tops);
     }
 }
 
diff --git a/src/rose/rose_build_misc.cpp b/src/rose/rose_build_misc.cpp
index b9aeabd0..dcb2a4eb 100644
--- a/src/rose/rose_build_misc.cpp
+++ b/src/rose/rose_build_misc.cpp
@@ -903,12 +903,15 @@ RoseVertex RoseBuildImpl::cloneVertex(RoseVertex v) {
 }
 
 #ifndef NDEBUG
-bool roseHasTops(const RoseGraph &g, RoseVertex v) {
+bool roseHasTops(const RoseBuildImpl &build, RoseVertex v) {
+    const RoseGraph &g = build.g;
     assert(g[v].left);
 
     set<u32> graph_tops;
-    for (const auto &e : in_edges_range(v, g)) {
-        graph_tops.insert(g[e].rose_top);
+    if (!build.isRootSuccessor(v)) {
+        for (const auto &e : in_edges_range(v, g)) {
+            graph_tops.insert(g[e].rose_top);
+        }
     }
 
     return is_subset_of(graph_tops, all_tops(g[v].left));
@@ -1073,18 +1076,9 @@ bool has_non_eod_accepts(const suffix_id &s) {
 set<u32> all_tops(const suffix_id &s) {
     assert(s.graph() || s.castle() || s.haig() || s.dfa());
     if (s.graph()) {
-        set<u32> tops;
-        const NGHolder &h = *s.graph();
-        for (const auto &e : out_edges_range(h.start, h)) {
-            if (target(e, h) == h.startDs) {
-                continue;
-            }
-            tops.insert(h[e].top);
-        }
-        if (tops.empty()) {
-            tops.insert(0); // Vacuous graph, triggered on zero top.
-        }
-        return tops;
+        flat_set<u32> tops = getTops(*s.graph());
+        assert(!tops.empty());
+        return {tops.begin(), tops.end()};
     }
 
     if (s.castle()) {
@@ -1142,18 +1136,8 @@ depth findMaxWidth(const left_id &r) {
 set<u32> all_tops(const left_id &r) {
     assert(r.graph() || r.castle() || r.haig() || r.dfa());
     if (r.graph()) {
-        set<u32> tops;
-        const NGHolder &h = *r.graph();
-        for (const auto &e : out_edges_range(h.start, h)) {
-            if (target(e, h) == h.startDs) {
-                continue;
-            }
-            tops.insert(h[e].top);
-        }
-        if (tops.empty()) {
-            tops.insert(0); // Vacuous graph, triggered on zero top.
-        }
-        return tops;
+        flat_set<u32> tops = getTops(*r.graph());
+        return {tops.begin(), tops.end()};
     }
 
     if (r.castle()) {
@@ -1348,6 +1332,49 @@ bool canImplementGraphs(const RoseBuildImpl &tbi) {
 
     return true;
 }
+
+bool hasOrphanedTops(const RoseBuildImpl &build) {
+    const RoseGraph &g = build.g;
+
+    ue2::unordered_map<left_id, set<u32> > roses;
+    ue2::unordered_map<suffix_id, set<u32> > suffixes;
+
+    for (auto v : vertices_range(g)) {
+        if (g[v].left) {
+            set<u32> &tops = roses[g[v].left];
+            if (!build.isRootSuccessor(v)) {
+                // Tops for infixes come from the in-edges.
+                for (const auto &e : in_edges_range(v, g)) {
+                    tops.insert(g[e].rose_top);
+                }
+            }
+        }
+        if (g[v].suffix) {
+            suffixes[g[v].suffix].insert(g[v].suffix.top);
+        }
+    }
+
+    for (const auto &e : roses) {
+        if (all_tops(e.first) != e.second) {
+            DEBUG_PRINTF("rose tops (%s) don't match rose graph (%s)\n",
+                         as_string_list(all_tops(e.first)).c_str(),
+                         as_string_list(e.second).c_str());
+            return true;
+        }
+    }
+
+    for (const auto &e : suffixes) {
+        if (all_tops(e.first) != e.second) {
+            DEBUG_PRINTF("suffix tops (%s) don't match rose graph (%s)\n",
+                         as_string_list(all_tops(e.first)).c_str(),
+                         as_string_list(e.second).c_str());
+            return true;
+        }
+    }
+
+    return false;
+}
+
 #endif // NDEBUG
 
 } // namespace ue2
diff --git a/src/rose/rose_build_role_aliasing.cpp b/src/rose/rose_build_role_aliasing.cpp
index b223fa92..66e44b3e 100644
--- a/src/rose/rose_build_role_aliasing.cpp
+++ b/src/rose/rose_build_role_aliasing.cpp
@@ -863,7 +863,7 @@ void pruneUnusedTops(CastleProto &castle, const RoseGraph &g,
 static
 void pruneUnusedTops(NGHolder &h, const RoseGraph &g,
                      const set<RoseVertex> &verts) {
-    ue2::unordered_set<u32> used_tops;
+    ue2::flat_set<u32> used_tops;
     for (auto v : verts) {
         assert(g[v].left.graph.get() == &h);
 
@@ -879,10 +879,13 @@ void pruneUnusedTops(NGHolder &h, const RoseGraph &g,
         if (v == h.startDs) {
             continue; // stylised edge, leave it alone.
         }
-        u32 top = h[e].top;
-        if (!contains(used_tops, top)) {
-            DEBUG_PRINTF("edge (start,%u) has unused top %u\n",
-                          h[v].index, top);
+        flat_set<u32> pruned_tops;
+        auto pt_inserter = inserter(pruned_tops, pruned_tops.end());
+        set_intersection(h[e].tops.begin(), h[e].tops.end(),
+                         used_tops.begin(), used_tops.end(), pt_inserter);
+        h[e].tops = move(pruned_tops);
+        if (h[e].tops.empty()) {
+            DEBUG_PRINTF("edge (start,%u) has only unused tops\n", h[v].index);
             dead.push_back(e);
         }
     }
@@ -1327,8 +1330,8 @@ bool attemptRoseGraphMerge(RoseBuildImpl &build, bool preds_same, RoseVertex a,
     DEBUG_PRINTF("winner %zu states\n", num_vertices(*b_h));
 
     if (!setDistinctRoseTops(g, victim, *b_h, deque<RoseVertex>(1, a))) {
-        assert(roseHasTops(g, a));
-        assert(roseHasTops(g, b));
+        assert(roseHasTops(build, a));
+        assert(roseHasTops(build, b));
         return false;
     }
 
@@ -1341,8 +1344,8 @@ bool attemptRoseGraphMerge(RoseBuildImpl &build, bool preds_same, RoseVertex a,
         for (const auto &e : in_edges_range(a, g)) {
             g[e] = a_props[source(e, g)];
         }
-        assert(roseHasTops(g, a));
-        assert(roseHasTops(g, b));
+        assert(roseHasTops(build, a));
+        assert(roseHasTops(build, b));
         return false;
     }
 
@@ -1365,8 +1368,8 @@ bool attemptRoseGraphMerge(RoseBuildImpl &build, bool preds_same, RoseVertex a,
 
     reduceImplementableGraph(*b_h, SOM_NONE, nullptr, build.cc);
 
-    assert(roseHasTops(g, a));
-    assert(roseHasTops(g, b));
+    assert(roseHasTops(build, a));
+    assert(roseHasTops(build, b));
     assert(isImplementableNFA(*b_h, nullptr, build.cc));
     return true;
 }
@@ -1417,8 +1420,8 @@ bool attemptRoseMerge(RoseBuildImpl &build, bool preds_same, RoseVertex a,
         return false;
     }
 
-    assert(roseHasTops(g, a));
-    assert(roseHasTops(g, b));
+    assert(roseHasTops(build, a));
+    assert(roseHasTops(build, b));
 
     if (a_left_id.graph() && b_left_id.graph()) {
         return attemptRoseGraphMerge(build, preds_same, a, b, trivialCasesOnly,
@@ -1737,6 +1740,7 @@ void leftMergePass(CandidateSet &candidates, RoseBuildImpl &build,
     }
 
     DEBUG_PRINTF("%zu candidates remaining\n", candidates.size());
+    assert(!hasOrphanedTops(build));
 }
 
 // Can't merge vertices with different root predecessors.
@@ -1952,6 +1956,7 @@ void rightMergePass(CandidateSet &candidates, RoseBuildImpl &build,
     }
 
     DEBUG_PRINTF("%zu candidates remaining\n", candidates.size());
+    assert(!hasOrphanedTops(build));
 }
 
 /**
@@ -2017,6 +2022,7 @@ void filterDiamondCandidates(RoseGraph &g, CandidateSet &candidates) {
 void aliasRoles(RoseBuildImpl &build, bool mergeRoses) {
     const CompileContext &cc = build.cc;
     RoseGraph &g = build.g;
+    assert(!hasOrphanedTops(build));
 
     if (!cc.grey.roseRoleAliasing || !cc.grey.roseGraphReduction) {
         return;
@@ -2050,6 +2056,7 @@ void aliasRoles(RoseBuildImpl &build, bool mergeRoses) {
 
     DEBUG_PRINTF("killed %zu vertices\n", dead.size());
     build.removeVertices(dead);
+    assert(!hasOrphanedTops(build));
 }
 
 } // namespace ue2
diff --git a/src/util/ue2_containers.h b/src/util/ue2_containers.h
index b6425f77..5bbf4cfe 100644
--- a/src/util/ue2_containers.h
+++ b/src/util/ue2_containers.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, Intel Corporation
+ * Copyright (c) 2015-2016, Intel Corporation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met: