(defun patch (func pattern patch)
  "Patch a function definition by replacing `pattern' by `patch'."
  (and (byte-code-function-p (symbol-function func))
    (error "Symbol `%s' is bound to a byte compiled function." func))
  (fset func (repl (symbol-function func) pattern patch)))

(defun repl (exp pattern patch)
  (cond
    ((null exp) nil)
    ((listp exp)
      (let ((expr (repl (car exp) pattern patch)))
        (cons
          (if (equal expr pattern) patch expr)
          (repl (cdr exp) pattern patch))))
      (t exp)))