def collate(self, merge_type=None, drop=[], drop_constant=False):
        from .element import Collator
        merge_type=merge_type if merge_type else self.__class__
        return Collator(self, merge_type=merge_type, drop=drop,
                        drop_constant=drop_constant)()
Collation allows collapsing nested HoloMaps by merging
dataset/ETHPy150Open ioam/holoviews/holoviews/core/spaces.py/HoloMap.collate