"a(ab|ba)+aa*b"
(define (acc1 lst)
  (cond ((null? lst) #f)
        ((and (eq? (car lst) 'a) (eq? (cadr lst) 'a) (eq? (caddr lst) 'b) (null? (cdddr lst))) #f)
        ((eq? (car lst) 'a) (acc2 (cdr lst)))
        ))
(define (acc2 lst)
  (cond ((null? lst) #f)
        ((null? (cdr lst)) #f)
        ((and (eq? (car lst) 'a) (eq? (cadr lst) 'a)) (acc3 (cddr lst)))
        ((and (eq? (car lst) 'a) (eq? (cadr lst) 'b) (null? (cddr lst))) #t)
        ((and (eq? (car lst) 'a) (eq? (cadr lst) 'b)) (acc2 (cddr lst)))
        ((and (eq? (car lst) 'b) (eq? (cadr lst) 'a)) (acc2 (cddr lst)))
        (else #f)
        ))
(define (acc3 lst)
  (cond ((null? lst) #f)
        ((eq? (car lst) 'a) (acc3 (cdr lst)))
        ((and (eq? (car lst) 'b) (null? (cdr lst))) #t)
        (else #f)))