#lang scheme

(provide if-differ)

#| If two expressions evaluate to the same (equal?) value,
    produce that value,
    otherwise evaluate the third expression.

   (if-differ <e1> <e2> <result-if-differ>) |#

(define-syntax-rule (if-differ <e1> <e2> 
                               <result-if-differ>)
  (let ([e1 <e1>])
    (if (equal? e1 <e2>)
        e1
        <result-if-differ>)))