colorize empty set

brag/private/colorer.rkt

@@ -11,6 +11,7 @@
    [(:or (:: "\"" "\\" "\"" "\"") ; string containg double-quote = "\""
          (from/to "'" "'")
          (from/to "\"" "\"")) (token 'LIT lexeme)]
+   [(:or "()" "Ø" "∅") (token 'NO-COLOR lexeme)] ; empty set symbols
    [(:or (char-set "()[]{}|+*:?") hide-char splice-char) (token 'MISC lexeme)]
    [(:seq (:or "#" ";") (complement (:seq (:* any-char) NL (:* any-char))) (:or NL "")) (token 'COMMENT lexeme)]
    [id (token 'ID lexeme)]