code: func (e *ErrorClass) MustAddData(key DataKey, value interface{}) {
  	if _, ex := e.data[key]; ex {
  		panic("key already exists")
  	}
  	e.data[key] = value
  }
docs: // MustAddData allows adding data key value pairs to error classes after they
  // are created. This is useful for allowing external packages add namespaced
  // values to errors defined outside of their package. It will panic if the
  // key is already set in the error class.