def labeller(landmarkable, group, label_func):
    new_group = label_func(landmarkable.landmarks[group])
    landmarkable.landmarks[label_func.group_label] = new_group
    return landmarkable
Re-label an existing landmark group on a :map:`Landmarkable` object with a
dataset/ETHPy150Open menpo/menpo/menpo/landmark/labels/base.py/labeller