gorilla.go
func (m *GorillaPathPrefix) Build(u *url.URL, values url.Values) error {
	path, err := m.RevertValid(values)
	if err == nil {
		u.Path = path
	}
	return err
}
func (m *GorillaPathPrefix) Build(u *url.URL, values url.Values) error {
	path, err := m.RevertValid(values)
	if err == nil {
		u.Path = path
	}
	return err
}
// Build builds the URL path using the given positional and named variables,
// and writes it to the given URL.