diff --git a/harmony_model_checker/charm/graph.h b/harmony_model_checker/charm/graph.h
index f72a343f..c446650b 100644
--- a/harmony_model_checker/charm/graph.h
+++ b/harmony_model_checker/charm/graph.h
@@ -44,6 +44,11 @@ struct access_info {
 // but, for memory efficiency, not the details of the microsteps themselves.  If
 // a failure is found, that information is recovered by re-executing the path to
 // the faulty state.
+//
+// TODO.  Can replace ctx with ctx_index into src->state, but should also
+//        support, say, -1 for inv_context.  after could be replaced with
+//        an index into dst->state.  choice could be the first entry in log
+//        Doing all three could save around 20 bytes per edge.
 struct edge {
     struct edge *fwdnext;    // forward linked list maintenance
     hvalue_t ctx, choice;    // ctx that made the microstep, choice if any
diff --git a/harmony_model_checker/charm/ops.c b/harmony_model_checker/charm/ops.c
index f028d08d..704eb394 100644
--- a/harmony_model_checker/charm/ops.c
+++ b/harmony_model_checker/charm/ops.c
@@ -1374,6 +1374,7 @@ void op_Go(
     struct context *copy = (struct context *) buffer;
     ctx_push(copy, result);
     copy->stopped = false;
+    // TODO.  Check success of context_add
     context_add(state, value_put_context(&step->engine, copy));
 #ifdef HEAP_ALLOC
     free(buffer);
@@ -2136,7 +2137,7 @@ void op_Spawn(
     }
     else {
         hvalue_t context = value_put_context(&step->engine, ctx);
-        if (!context_add(state, context)) {
+        if (context_add(state, context) < 0) {
             value_ctx_failure(step->ctx, &step->engine, "spawn: too many threads");
             return;
         }