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