Pattern matching can be used to determine whether a function matches a design pattern:

(define pattern
  '(define (?fun tree)
     (if (pair? tree)
         (?combine (?fun (car tree))
                   (?fun (cdr tree)))
         (if (?test? tree) ?trueval ?falseval))))

> (match? pattern
          '(define (nnums tree)
             (if (pair? tree)
                 (+ (nnums (car tree))
                    (nnums (cdr tree)))
             (if (number? tree) 1 0))) )

((?falseval . 0) (?trueval . 1)
 (?test? . number?) (?combine . +)
 (?fun . nnums))

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