diff --git a/lex.ml b/lex.ml
index d155eaa..0d8fed7 100644
--- a/lex.ml
+++ b/lex.ml
@@ -9,6 +9,8 @@ let expected col c = raise @@ Expected (col, c)
 let either f g c =
   f c || g c
 
+(* test functions *)
+
 let is_digit c =
   '0' <= c && c <= '9'
 
@@ -30,10 +32,11 @@ let is_ident_start =
 let is_ident =
   either is_ident_start is_digit
 
+(* expect_char expects c at col of seq. If not, raise Expected. *)
 let expect_char col c seq =
   match seq () with
   | Seq.Nil -> expected col c
-  | Seq.Cons ((_, x), seq) ->
+  | Seq.Cons ((col, x), seq) ->
     if x = c then seq else expected col c
 
 let expect_token str tok seq =