Implement the first substitute()
diff --git a/src/hb-ot-layout-open-private.h b/src/hb-ot-layout-open-private.h
index d5ca810..4123a43 100644
--- a/src/hb-ot-layout-open-private.h
+++ b/src/hb-ot-layout-open-private.h
@@ -671,7 +671,7 @@
   inline bool ignore_base_glyphs(void) const { return lookupFlag & LookupFlag::IgnoreBaseGlyphs; }
   inline bool ignore_ligatures	(void) const { return lookupFlag & LookupFlag::IgnoreLigatures; }
   inline bool ignore_marks	(void) const { return lookupFlag & LookupFlag::IgnoreMarks; }
-  inline bool get_mark_attachment_type (void) const { return lookupFlag & LookupFlag::MarkAttachmentType; }
+  inline unsigned int get_mark_attachment_type (void) const { return lookupFlag & LookupFlag::MarkAttachmentType; }
 
   inline unsigned int get_type (void) const { return lookupType; }
   inline unsigned int get_flag (void) const { return lookupFlag; }