let symbol_of s = List.assoc s
     [
      ","COMMA";"SEMICOLON":"COLON"."DOT;
      "{"LBRACE"}"RBRACE"["LBRACKET"]"RBRACKET;
      "("LPAREN")"RPAREN;
      "="EQUAL"?"QUESTION"|"VBAR"^"HAT"&"AMPERSAND;
      "<"LANGLE">"RANGLE;
      "+"PLUS"-"MINUS"*"STAR"/"SLASH"%"PERCENT;
      "~"TILDE"!"EXCL
    ]