def __getitem__(self, entity_class):
		"""Return an :class:`EntityExtent` for the given entity class. This extent
		can be used to access the set of entities of that class in the world
		or to query these entities via their components. 

		Examples::

			world[MyEntity]
			world[...]

		:param entity_class: The entity class for the extent.

			May also be a tuple of entity classes, in which case
			the extent returned contains union of all entities of the classes
			in the world.

			May also be the special value ellipsis (``...``), which
			returns an extent containing all entities in the world.  This allows
			you to conveniently query all entities using ``world[...]``.
		"""
		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 __HOLE__:
			extent = self._extents[entity_class] = EntityExtent(self, set())
			return extent
dataset/ETHPy150Open caseman/grease/grease/world.py/World.__getitem__