diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index 517f745f0..1baa9af5c 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -549,6 +549,8 @@ hb_buffer_t::delete_glyph ()
     goto done;
   }
 
+  /* TODO We need to just merge one direction, with whichever is smaller. */
+
   if (out_len)
   {
     /* Merge cluster backward. */