code: func stripLeaveSlash(prefix string, h http.Handler) http.Handler {
  	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
  		p := strings.TrimPrefix(req.URL.Path, prefix)
  		if len(p) >= len(req.URL.Path) {
  			http.NotFound(w, req)
  			return
  		}
  		if len(p) > 0 && p[:1] != "/" {
  			p = "/" + p
  		}
  		req.URL.Path = p
  		h.ServeHTTP(w, req)
  	})
  }
docs: // like http.StripPrefix, but always leaves an initial slash. (so that our
  // regexps will work.)