Exercise 2.75.  Implement the constructor make-from-mag-ang in 
message-passing style. This procedure should be analogous to the 
make-from-real-imag procedure given above.

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(define (make-from-mag-ang mag ang)
  (lambda (op)
    (cond ((eq? op 'real-part) (* mag (cos ang)))
          ((eq? op 'imag-part) (* mag (sin ang)))
          ((eq? op 'magnitude) mag)
          ((eq? op 'angle) ang)
          (else (error "Listen, broccoli brains, I don't have time for this trash.")))))