code: func (s SimpleRule) Rewrite(fs http.FileSystem, r *http.Request) Result {
  	// take note of this rewrite for internal use by fastcgi
  	// all we need is the URI, not full URL
  	r.Header.Set(headerFieldName, r.URL.RequestURI())
  
  	// attempt rewrite
  	return To(fs, r, s.To, newReplacer(r))
  }
docs: // Rewrite rewrites the internal location of the current request.