Better.
(define (mk-q) '())(define (en-q q x) (cons x q))(define (de-q q) (if (empty? q) 'error (cdr q)))(define (head q) (if (empty? q) 'error (car q)))(define (size q) (if (empty? q) 0 (+ 1 (size (de-q q))))) (define (empty? q) (null? q))