class Array
	def all_perm
		return yield [] if empty?
		cur = pop
		post = []
		while cur
			(self + post).all_perm { |rest|
				yield rest << cur
			}
			post.unshift cur
			cur = pop
		end
		replace post
	end
end

if $0 == __FILE__
a = [1, 2, 3, 1]
a.all_perm { |s| p s }
puts
p a
end