; Evaluation of IF special form
(define (m-if ifform env)
  (if (m-eval (cadr ifform) env)
      (m-eval (caddr ifform) env)
      (m-eval (cadddr ifform) env)))


; Evaluation of SET! special form:
; change the value in the environment
(define (m-set! form env)
  (let ((pair (assoc (cadr form) env)))
    (if pair
        (set-cdr! pair
                  (m-eval (caddr form) env))
        (m-error "Unbound variable " (cadr form))) ) )


; Evaluation of BEGIN special form
(define (m-begin statements env lastval)
  (if (null? statements)
      lastval
      (m-begin (cdr statements)
               env
               (m-eval (car statements) env)) ) )

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