sublis can be used to write code by instantiating design patterns. For example, we can instantiate a test-accumulate pattern to make various counting functions:

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

>(sublis '((?fun . nsymbols)
           (?combine . +)
           (?test? . symbol?)
           (?trueval . 1)
           (?falseval . 0))
         pattern)

(define (nsymbols tree)
  (if (pair? tree)
      (+ (nsymbols (car tree))
         (nsymbols (cdr tree)))
      (if (symbol? tree) 1 0)))