+ | "array" -> Some ARRAY
+ | "break" -> Some BREAK
+ | "do" -> Some DO
+ | "else" -> Some ELSE
+ | "end" -> Some END
+ | "for" -> Some FOR
+ | "function" -> Some FUNCTION
+ | "if" -> Some IF
+ | "in" -> Some IN
+ | "let" -> Some LET
+ | "nil" -> Some NIL
+ | "of" -> Some OF
+ | "then" -> Some THEN
+ | "to" -> Some TO
+ | "type" -> Some TYPE
+ | "var" -> Some VAR
+ | "while" -> Some WHILE
+ | _ -> Some (ID id)