[OT] Implement serialize() for MultiSubst
diff --git a/src/hb-open-type-private.hh b/src/hb-open-type-private.hh
index 11cb621..384cdb7 100644
--- a/src/hb-open-type-private.hh
+++ b/src/hb-open-type-private.hh
@@ -677,6 +677,16 @@
   { return len.static_size + len * Type::static_size; }
 
   inline bool serialize (hb_serialize_context_t *c,
+			 unsigned int items_len)
+  {
+    TRACE_SERIALIZE ();
+    if (unlikely (!c->extend_min (*this))) return TRACE_RETURN (false);
+    len.set (items_len); /* TODO(serialize) Overflow? */
+    if (unlikely (!c->extend (*this))) return TRACE_RETURN (false);
+    return TRACE_RETURN (true);
+  }
+
+  inline bool serialize (hb_serialize_context_t *c,
 			 const Type *items,
 			 unsigned int items_len)
   {