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       # MATH build up functions of several variables
[hear] (= ((? x / ? y / - (x) (y)) 3 3) 0);

[hear] (= ((? x / ? y / - (x) (y)) 8 7) 1);

[hear] (= ((? x / ? y / - (x) (y)) 13 9) 4);

[hear] (= ((? x / ? y / - (x) (y)) 15 9) 6);

[hear] (= ((? x / ? y / - (x) (y)) 8 5) 3);

[hear] (define last /
         ? x /
         list-ref (x) (- (list-length / x) 1));

[hear] (define except-last /
         ? x /
         if (> (list-length / x) 1)
          (prepend
            (head /
             x)
            (except-last /
             tail /
             x))
          (vector));

       # test last and except-last
[hear] (= 15 (last / vector 4 5 15));

[hear] (list= (vector 4 5)
               (except-last /
                vector 4 5 15));

[hear] (intro lambda);

[hear] (define prev-translate / translate);

[hear] (define translate /
         let ((prev (prev-translate)))
          (? x /
           if (number? /
             x)
            (prev /
             x)
            (if (= (head / x) lambda)
              (let ((formals (head / tail / x))
                    (body (head / tail / tail / x)))
                (if (> (list-length / formals) 0)
                  (translate
                    (vector
                      lambda
                      (except-last /
                       formals)
                      (vector ? (last / formals) (body))))
                  (translate (body))))
              (prev /
               x))));

       # test lambda
[hear] (= (? x / - (x) 5) (lambda (x) (- (x) 5)));

[hear] (= (? x /
            ? y /
            - (x) (y))
           (lambda (x y) (- (x) (y))));

[hear] (= ((lambda (x y) (- (x) (y))) 9 1) 8);

[hear] (= ((lambda (x y) (- (x) (y))) 15 7) 8);

[hear] (= ((lambda (x y) (- (x) (y))) 6 0) 6);

[hear] (= ((lambda (x y) (- (x) (y))) 11 8) 3);

[hear] (= ((lambda (x y) (- (x) (y))) 6 6) 0);

[hear] (define apply /
         lambda (x y)
          (if (list= (y) (vector))
            (x)
            (apply ((x) (head / y)) (tail / y))));

[hear] (= (apply (lambda (x y) (- (x) (y))) (vector 8 1))
           7);

[hear] (= (apply (lambda (x y) (- (x) (y))) (vector 11 5))
           6);

[hear] (= (apply (lambda (x y) (- (x) (y))) (vector 8 4))
           4);

[hear] (= (apply (lambda (x y) (- (x) (y))) (vector 13 5))
           8);

[hear] (= (apply (lambda (x y) (- (x) (y))) (vector 9 4))
           5);