diff --git a/pitfall/fontkit/subset.rkt b/pitfall/fontkit/subset.rkt
index 56540788..cc9ac830 100644
--- a/pitfall/fontkit/subset.rkt
+++ b/pitfall/fontkit/subset.rkt
@@ -57,6 +57,11 @@ https://github.com/mbutterick/fontkit/blob/master/src/subset/TTFSubset.js
   (index? . ->m . index?)
 
   (define glyph (send (· this font) getGlyph gid))
+  ;; _decode unpacks the `glyf` table data corresponding to a certin gid.
+  ;; here, it's not necessary for non-composite glyphs
+  ;; because they just get copied entirely into the subset.
+  ;; it's just used to detect composite glyphs and handle them specially.
+  ;; so an optimization would be to detect composite / noncomposite without full _decode.
   (define glyf (send glyph _decode))
 
   ;; get the offset to the glyph from the loca table