/* Note that GPOS sets recurse_func to NULL already, so it doesn't get
* past the previous check. For GSUB, we only want to collect the output
- * glyphs in the recursion. If output is not requested, we can go home now. */
+ * glyphs in the recursion. If output is not requested, we can go home now.
+ *
+ * Note further, that the above is not exactly correct. A recursed lookup
+ * is allowed to match input that is not matched in the context, but that's
+ * not how most fonts are built. It's possible to relax that and recurse
+ * with all sets here if it proves to be an issue.
+ */
if (output == hb_set_get_empty ())
return HB_VOID;