diff --git a/spectec/doc/Language.md b/spectec/doc/Language.md
index 64ce0be564..4d13cd0def 100644
--- a/spectec/doc/Language.md
+++ b/spectec/doc/Language.md
@@ -75,6 +75,7 @@ iter ::=
   "*"                                  list
   "+"                                  non-empty list
   "^" arith                            list of specific length
+  "^" "(" id "<" arith ")"             list of specific length with index (only in expressions)
 ```
 Iterated types essentially describe options or lists of phrases.