let identifier_of s = List.assoc s 
     [
      "auto"AUTO;
      "register"REGISTER;
      "static"STATIC;
      "inline"INLINE;
      "extern"EXTERN;
      "typedef"TYPEDEF;
      "void"VOID;
      "char"CHAR;
      "short"SHORT;
      "int"INT;
      "long"LONG;
      "float"FLOAT;
      "double"DOUBLE;
      "signed"SIGNED;
      "unsigned"UNSIGNED;
      "const"CONST;
      "volatile"VOLATILE;
      "struct"STRUCT;
      "union"UNION;
      "enum"ENUM;
      "if"IF;
      "else"ELSE;
      "switch"SWITCH;
      "while"WHILE;
      "do"DO;
      "for"FOR;
      "goto"GOTO;
      "continue"CONTINUE;
      "break"BREAK;
      "return"RETURN;
      "sizeof"SIZEOF;
    ]