code: func MustGet(req *http.Request, param string) string {
  	if !IsGet(req) {
  		panic(InvalidMethodError{})
  	}
  	v := req.FormValue(param)
  	if v == "" {
  		panic(MissingParameterError(param))
  	}
  	return v
  }
docs: // MustGet returns a non-empty GET (or HEAD) parameter param and panics
  // with a special error as caught by a deferred httputil.Recover.