def __getitem__(self, entity_class):
		if isinstance(entity_class, tuple):
			entities = set()
			for cls in entity_class:
				if cls in self._extents:
					entities |= self._extents[cls].entities
			return EntityExtent(self, entities)
		elif entity_class is Ellipsis:
			return self._full_extent
		try:
			return self._extents[entity_class]
		except KeyError:
			extent = self._extents[entity_class] = EntityExtent(self, set())
			return extent
Return an :class:`EntityExtent` for the given entity class. This extent
dataset/ETHPy150Open caseman/grease/grease/world.py/World.__getitem__