def METHOD_NAME(measurement: np.ndarray, this: gtsam.CustomFactor,
             values: gtsam.Values,
             jacobians: Optional[List[np.ndarray]]) -> np.ndarray:
    """Landmark Factor error function
    :param measurement: Landmark measurement, to be filled with `partial`
    :param this: gtsam.CustomFactor handle
    :param values: gtsam.Values
    :param jacobians: Optional list of Jacobians
    :return: the unwhitened error
    """
    key = this.keys()[0]
    pos = values.atVector(key)
    error = pos - measurement
    if jacobians is not None:
        jacobians[0] = I
    return error