# Are the element in the generated sequence unique?
	# Default `true`.
	#
	# When `true`, an element cannot be reused in the same sequence (no replacement).
	#
	# Elements are distinguished by their order in the base collection,
	# not by their intrinsic value or equality.
	#
	# ~~~~
	# var xs = [1, 1, 2]
	# var cxs = new CombinationCollection[Int](xs, 2)
	# cxs.are_unique = true
	# assert cxs.to_a == [[1,1], [1,2], [1, 2]]
	# cxs.are_unique = false
	# assert cxs.to_a == [[1,1], [1,1], [1,2], [1,1], [1,2], [2,2]]
	# ~~~~
	var are_unique = true is writable