def all_subsets(arr):
	"""Returns a list of all subsets of a given list.
	Mathematically: a set of all subsets of a set

	Recursive implementation.
	"""
	set_length = len(arr)
	if set_length == 0:
		return [[]]
	else:
		subsets = all_subsets(arr[1:])
		return subsets + [sub + [arr[0]] for sub in subsets]