diff --git a/pdf.c b/pdf.c
index 9b58c23b05ebddfd48f377591bd411aa871f6902..2729f9a4718c973e826e9db1b385e75c41b7f1e9 100644
--- a/pdf.c
+++ b/pdf.c
@@ -662,6 +662,8 @@ init_parser(struct Env *aux)
 	H_RULE(xr_td,	SEQ(xrefs, KW("trailer"), ws, dict));
 
 	H_RULE(tail,	SEQ(body, h_optional(xr_td), startxr));
+		// XXX the real world likes to omit 'startxr' from all but the
+		// last trailer. we should accept-and-warn in that case.
 	H_RULE(pdf,	SEQ(header, h_many1(tail), end));
 
 	/* debug parser to consume as much as possible */