recursive power set

  # Let's suppose that there is a method Set.pickOne()
  # which picks randomly an element from a given set
  # We also introduce Set.empty which is the neutral element for the union operation (+)
	function power_set(items)
		if items.empty
			return Set.empty
		set <- copy(items)
		e <- set.pickOne()
		ps <- power_set(set.remove(e))
		ps <- ps + add(ps, e)
		return ps

	# Iterative add
	# arguments:
	# 	set_of_sets: a set of sets
	#   e: an element to be added to each set inside set_of_sets
	function add(set_of_sets, e)
		set <- copy(set_of_sets)
		foreach subset in set
		    subset.add(e)
		return set

	# Recursive add: this suppose the existence of a constructor Set.new(element) 
	# to create a set with an initial element
	function add(set_of_sets, e)
		if set_of_sets.empty 
			return Set.empty
		subsets <- copy(set_of_sets)
		subset <- subsets.pickOne()
		subsets.remove(subset)
		subset.add(e)
		return Set.new(subset) + add(subsets, e)