#lang racket
;;; BNF of LON:
;;; <list-of-numbers> ::= ()
;;;                   ::= (<number> . <list-of-numbers>)

(define lon?
  (lambda (l)
    (cond [(null? l)]
          [(pair? l) (and (number? (car l))
                          (lon? (cdr l)))]
          [else #f])))
        

;;; BNF of (simple) expression:
;;; <expression> ::= <identifier>
;;;              ::= (lambda (<identifier>) <expression>)
;;;              ::= (<express(expression> <expression>)

(define expression?
  (lambda (exp)
    (cond [(symbol? exp)]
          [(pair? exp) (if (eq? (car exp) 'lambda)
                           (and (pair? (cadr exp))
                                (symbol? (caadr exp))
                                (expression? (caddr exp)))
                           (and (expression? (car exp))
                                (expression? (cadr exp))))]
          [else #f])))