diff --git a/src/nfa/callback.h b/src/nfa/callback.h
index 0284f1d5..9bdaa8d1 100644
--- a/src/nfa/callback.h
+++ b/src/nfa/callback.h
@@ -37,24 +37,26 @@
 
 /** \brief The type for an NFA callback.
  *
- * This is a function that takes as arguments the current offset where the
- * match occurs, the id of the match and the context pointer that was passed
- * into the NFA API function that executed the NFA.
+ * This is a function that takes as arguments the current start and end offsets
+ * where the match occurs, the id of the match and the context pointer that was
+ * passed into the NFA API function that executed the NFA.
  *
- * The offset where the match occurs will be the offset after the character
- * that caused the match. Thus, if we have a buffer containing 'abc', then a
- * pattern that matches an empty string will have an offset of 0, a pattern
- * that matches 'a' will have an offset of 1, and a pattern that matches 'abc'
- * will have an offset of 3, which will be a value that is 'beyond' the size of
- * the buffer. That is, if we have n characters in the buffer, there are n+1
- * different potential offsets for matches.
+ * The start offset is the "start of match" (SOM) offset for the match. It is
+ * only provided by engines that natively support SOM tracking (e.g. Gough).
+ *
+ * The end offset will be the offset after the character that caused the match.
+ * Thus, if we have a buffer containing 'abc', then a pattern that matches an
+ * empty string will have an offset of 0, a pattern that matches 'a' will have
+ * an offset of 1, and a pattern that matches 'abc' will have an offset of 3,
+ * which will be a value that is 'beyond' the size of the buffer. That is, if
+ * we have n characters in the buffer, there are n+1 different potential
+ * offsets for matches.
  *
  * This function should return an int - currently the possible return values
  * are 0, which means 'stop running the engine' or non-zero, which means
  * 'continue matching'.
  */
-typedef int (*NfaCallback)(u64a from_offset, u64a to_offset, ReportID id,
-                           void *context);
+typedef int (*NfaCallback)(u64a start, u64a end, ReportID id, void *context);
 
 /**
  * standard \ref NfaCallback return value indicating that engine execution
diff --git a/src/rose/catchup.c b/src/rose/catchup.c
index 9a075d17..017a6bf0 100644
--- a/src/rose/catchup.c
+++ b/src/rose/catchup.c
@@ -281,15 +281,14 @@ restart:
 
 /* for use by mpv (chained) only */
 static
-int roseNfaFinalBlastAdaptor(u64a som, u64a offset, ReportID id,
-                             void *context) {
+int roseNfaFinalBlastAdaptor(u64a start, u64a end, ReportID id, void *context) {
     struct hs_scratch *scratch = context;
+    assert(scratch && scratch->magic == SCRATCH_MAGIC);
     const struct RoseEngine *t = scratch->core_info.rose;
 
-    DEBUG_PRINTF("masky got himself a blasted match @%llu id %u !woot!\n",
-                 offset, id);
+    DEBUG_PRINTF("id=%u matched at [%llu,%llu]\n", id, start, end);
 
-    int cb_rv = roseNfaRunProgram(t, scratch, som, offset, id, 1);
+    int cb_rv = roseNfaRunProgram(t, scratch, start, end, id, 1);
     if (cb_rv == MO_HALT_MATCHING) {
         return MO_HALT_MATCHING;
     } else if (cb_rv == ROSE_CONTINUE_MATCHING_NO_EXHAUST) {
@@ -449,35 +448,35 @@ char in_mpv(const struct RoseEngine *rose, const struct hs_scratch *scratch) {
 }
 
 static
-int roseNfaBlastAdaptor(u64a som, u64a offset, ReportID id, void *context) {
+int roseNfaBlastAdaptor(u64a start, u64a end, ReportID id, void *context) {
     struct hs_scratch *scratch = context;
-    struct RoseContext *tctxt = &scratch->tctxt;
+    assert(scratch && scratch->magic == SCRATCH_MAGIC);
     const struct RoseEngine *t = scratch->core_info.rose;
 
-    DEBUG_PRINTF("masky got himself a blasted match @%llu id %u !woot!\n",
-                 offset, id);
+    DEBUG_PRINTF("id=%u matched at [%llu,%llu]\n", id, start, end);
 
     const char from_mpv = in_mpv(t, scratch);
-    int cb_rv = roseNfaRunProgram(t, scratch, som, offset, id, from_mpv);
+    int cb_rv = roseNfaRunProgram(t, scratch, start, end, id, from_mpv);
     if (cb_rv == MO_HALT_MATCHING) {
         return MO_HALT_MATCHING;
     } else if (cb_rv == ROSE_CONTINUE_MATCHING_NO_EXHAUST) {
         return MO_CONTINUE_MATCHING;
     } else {
         assert(cb_rv == MO_CONTINUE_MATCHING);
-        return !roseSuffixIsExhausted(t, tctxt->curr_qi,
+        return !roseSuffixIsExhausted(t, scratch->tctxt.curr_qi,
                                       scratch->core_info.exhaustionVector);
     }
 }
 
-int roseNfaAdaptor(u64a from_offset, u64a offset, ReportID id,
-                   void *context) {
+int roseNfaAdaptor(u64a start, u64a end, ReportID id, void *context) {
     struct hs_scratch *scratch = context;
-    DEBUG_PRINTF("masky got himself a match @%llu id %u !woot!\n", offset, id);
+    assert(scratch && scratch->magic == SCRATCH_MAGIC);
+
+    DEBUG_PRINTF("id=%u matched at [%llu,%llu]\n", id, start, end);
 
     /* must be a external report as haig cannot directly participate in chain */
-    return roseNfaRunProgram(scratch->core_info.rose, scratch, from_offset,
-                             offset, id, 0);
+    return roseNfaRunProgram(scratch->core_info.rose, scratch, start, end, id,
+                             0);
 }
 
 static really_inline
diff --git a/src/rose/match.c b/src/rose/match.c
index eb8def9b..2b05fd76 100644
--- a/src/rose/match.c
+++ b/src/rose/match.c
@@ -211,8 +211,9 @@ event_enqueued:
     return HWLM_CONTINUE_MATCHING;
 }
 
-int roseAnchoredCallback(u64a som, u64a end, u32 id, void *ctx) {
+int roseAnchoredCallback(u64a start, u64a end, u32 id, void *ctx) {
     struct hs_scratch *scratch = ctx;
+    assert(scratch && scratch->magic == SCRATCH_MAGIC);
     struct RoseContext *tctxt = &scratch->tctxt;
     struct core_info *ci = &scratch->core_info;
     const struct RoseEngine *t = ci->rose;
@@ -244,7 +245,7 @@ int roseAnchoredCallback(u64a som, u64a end, u32 id, void *ctx) {
     const u32 *programs = getByOffset(t, t->litProgramOffset);
     assert(id < t->literalCount);
     const u8 flags = ROSE_PROG_FLAG_IN_ANCHORED;
-    if (roseRunProgram(t, scratch, programs[id], som, real_end, match_len,
+    if (roseRunProgram(t, scratch, programs[id], start, real_end, match_len,
                        flags) == HWLM_TERMINATE_MATCHING) {
         assert(can_stop_matching(scratch));
         DEBUG_PRINTF("caller requested termination\n");
@@ -647,11 +648,12 @@ int roseRunBoundaryProgram(const struct RoseEngine *rose, u32 program,
     return MO_CONTINUE_MATCHING;
 }
 
-int roseReportAdaptor(u64a som, u64a offset, ReportID id, void *context) {
-    DEBUG_PRINTF("som=%llu, offset=%llu, id=%u\n", som, offset, id);
+int roseReportAdaptor(u64a start, u64a end, ReportID id, void *context) {
     struct hs_scratch *scratch = context;
     assert(scratch && scratch->magic == SCRATCH_MAGIC);
 
+    DEBUG_PRINTF("id=%u matched at [%llu,%llu]\n", id, start, end);
+
     const struct RoseEngine *rose = scratch->core_info.rose;
 
     // Our match ID is the program offset.
@@ -659,7 +661,7 @@ int roseReportAdaptor(u64a som, u64a offset, ReportID id, void *context) {
     const size_t match_len = 0; // Unused in this path.
     const u8 flags = ROSE_PROG_FLAG_SKIP_MPV_CATCHUP;
     hwlmcb_rv_t rv =
-        roseRunProgram(rose, scratch, program, som, offset, match_len, flags);
+        roseRunProgram(rose, scratch, program, start, end, match_len, flags);
     if (rv == HWLM_TERMINATE_MATCHING) {
         return MO_HALT_MATCHING;
     }
diff --git a/src/rose/match.h b/src/rose/match.h
index 49afa588..b69ff158 100644
--- a/src/rose/match.h
+++ b/src/rose/match.h
@@ -48,7 +48,7 @@
 
 /* Callbacks, defined in catchup.c */
 
-int roseNfaAdaptor(u64a from_offset, u64a offset, ReportID id, void *context);
+int roseNfaAdaptor(u64a start, u64a end, ReportID id, void *context);
 
 /* Callbacks, defined in match.c */
 
@@ -56,7 +56,7 @@ hwlmcb_rv_t roseCallback(size_t start, size_t end, u32 id, void *ctx);
 hwlmcb_rv_t roseFloatingCallback(size_t start, size_t end, u32 id, void *ctx);
 hwlmcb_rv_t roseDelayRebuildCallback(size_t start, size_t end, u32 id,
                                      void *ctx);
-int roseAnchoredCallback(u64a som, u64a end, u32 id, void *ctx);
+int roseAnchoredCallback(u64a start, u64a end, u32 id, void *ctx);
 
 /* Common code, used all over Rose runtime */
 
diff --git a/src/rose/program_runtime.c b/src/rose/program_runtime.c
index 7669103f..23532d40 100644
--- a/src/rose/program_runtime.c
+++ b/src/rose/program_runtime.c
@@ -33,10 +33,11 @@
 
 #include "program_runtime.h"
 
-int roseNfaEarliestSom(u64a from_offset, UNUSED u64a offset, UNUSED ReportID id,
+int roseNfaEarliestSom(u64a start, UNUSED u64a end, UNUSED ReportID id,
                        void *context) {
+    assert(context);
     u64a *som = context;
-    *som = MIN(*som, from_offset);
+    *som = MIN(*som, start);
     return MO_CONTINUE_MATCHING;
 }
 
diff --git a/src/rose/program_runtime.h b/src/rose/program_runtime.h
index e90395fb..fe71772e 100644
--- a/src/rose/program_runtime.h
+++ b/src/rose/program_runtime.h
@@ -702,8 +702,8 @@ int roseCheckLookaround(const struct RoseEngine *t,
     return 1;
 }
 
-int roseNfaEarliestSom(u64a from_offset, u64a offset, ReportID id,
-                       void *context);
+int roseNfaEarliestSom(u64a start, u64a end, ReportID id, void *context);
+
 static rose_inline
 u64a roseGetHaigSom(const struct RoseEngine *t, struct hs_scratch *scratch,
                     const u32 qi, UNUSED const u32 leftfixLag) {
diff --git a/src/rose/rose.h b/src/rose/rose.h
index ecf16854..280e3bd5 100644
--- a/src/rose/rose.h
+++ b/src/rose/rose.h
@@ -49,7 +49,7 @@ void roseStreamEodExec(const struct RoseEngine *t, u64a offset,
 hwlmcb_rv_t rosePureLiteralCallback(size_t start, size_t end, u32 id,
                                     void *context);
 
-int roseReportAdaptor(u64a som, u64a offset, ReportID id, void *context);
+int roseReportAdaptor(u64a start, u64a end, ReportID id, void *context);
 
 int roseRunBoundaryProgram(const struct RoseEngine *rose, u32 program,
                            u64a stream_offset, struct hs_scratch *scratch);
diff --git a/src/som/som_runtime.c b/src/som/som_runtime.c
index b9972b2c..1a868efc 100644
--- a/src/som/som_runtime.c
+++ b/src/som/som_runtime.c
@@ -87,14 +87,14 @@ char ok_and_mark_if_unset(u8 *som_store_valid, struct fatbit *som_set_now,
 }
 
 static
-int somRevCallback(UNUSED u64a som, u64a offset, ReportID id, void *ctx) {
-    DEBUG_PRINTF("offset=%llu, id=%u\n", offset, id);
+int somRevCallback(UNUSED u64a start, u64a end, ReportID id, void *ctx) {
+    DEBUG_PRINTF("offset=%llu, id=%u\n", end, id);
 
     // We use the id to store the offset adjustment (for assertions like a
     // leading \b or multiline mode).
     assert(id <= 1);
     u64a *from_offset = ctx;
-    LIMIT_TO_AT_MOST(from_offset, offset + id);
+    LIMIT_TO_AT_MOST(from_offset, end + id);
     return 1; // continue matching.
 }