code: func (r *RewriteOptions) treeFn() TreeCallbackFn {
  	if r.TreeCallbackFn == nil {
  		return noopTreeFn
  	}
  	return r.TreeCallbackFn
  }
docs: // treeFn returns a useable TreeRewriteFn, either the one that was given in the
  // *RewriteOptions, or a noopTreeFn.