Add all of the elements of other to self.

		if isinstance(it, _basebag):
				This runs in O(it.num_unique_elements())
		else:
				This runs in O(len(it))

def _iadd(self, other):
		"""Add all of the elements of other to self.

		if isinstance(it, _basebag):
				This runs in O(it.num_unique_elements())
		else:
				This runs in O(len(it))
		"""
		if isinstance(other, _basebag):
			for elem, count in other.counts():
				self._increment_count(elem, count)
		else:
			for elem in other:
				self._increment_count(elem, 1)
		return self