diff --git a/src/cfgrammar.c b/src/cfgrammar.c
index 196d9d3..b33ad86 100644
--- a/src/cfgrammar.c
+++ b/src/cfgrammar.c
@@ -281,6 +281,13 @@ void h_stringmap_update(HStringMap *m, const HStringMap *n)
   h_hashtable_merge(combine_stringmap, m->char_branches, n->char_branches);
 }
 
+HStringMap *h_stringmap_copy(HArena *a, const HStringMap *m)
+{
+  HStringMap *res = h_stringmap_new(a);
+  h_stringmap_update(res, m);
+  return res;
+}
+
 /* Replace all occurances of old in m with new.
  * If old is NULL, replace all values in m with new.
  * If new is NULL, remove the respective values.
diff --git a/src/cfgrammar.h b/src/cfgrammar.h
index 193f8ca..9cefc62 100644
--- a/src/cfgrammar.h
+++ b/src/cfgrammar.h
@@ -40,6 +40,7 @@ typedef struct HStringMap_ {
 } HStringMap;
 
 HStringMap *h_stringmap_new(HArena *a);
+HStringMap *h_stringmap_copy(HArena *a, const HStringMap *m);
 void h_stringmap_put_end(HStringMap *m, void *v);
 void h_stringmap_put_epsilon(HStringMap *m, void *v);
 void h_stringmap_put_after(HStringMap *m, uint8_t c, HStringMap *ends);