code: func (fb *Firebase) Shallow(v bool) {
  	fb.paramsMtx.Lock()
  	if v {
  		fb.params.Set(shallowParam, "true")
  	} else {
  		fb.params.Del(shallowParam)
  	}
  	fb.paramsMtx.Unlock()
  }
docs: // Shallow limits the depth of the data returned when calling Value.
  // If the data at the location is a JSON primitive (string, number or boolean),
  // its value will be returned. If the data is a JSON object, the values
  // for each key will be truncated to true.
  //
  // Reference https://firebase.google.com/docs/database/rest/retrieve-data#shallow