Clear all objects from the collections associated with the
			objects in `*cls`. **use with caution!**

def clear_collection(self, *classes):
		''' Clear all objects from the collections associated with the
			objects in `*cls`. **use with caution!**'''
		for c in classes:
			self.queue.append(ClearCollectionOp(self.transaction_id, self, c))
		if self.autoflush:
			self.flush()