code: func (fb *Firebase) OrderBy(value string) *Firebase {
  	c := fb.copy()
  	// explicitly not locking here because no one else can
  	// modify this value before we return it.
  	if value != "" {
  		c.params.Set(orderByParam, escapeString(value))
  	} else {
  		c.params.Del(orderByParam)
  	}
  	return c
  }
docs: // OrderBy creates a new Firebase reference with the
  // requested OrderBy configuration. The value that is passed in
  // is automatically escaped if it is a string value.
  //
  //    OrderBy("foo")   // -> orderBy="foo"
  //    OrderBy(`"foo"`) // -> orderBy="foo"
  //    OrderBy("$key")  // -> orderBy="$key"
  //
  // Reference https://firebase.google.com/docs/database/rest/retrieve-data#orderby