# All `r`-length combinations on self (in same order) without repeated elements.
	#
	# See `CartesianCollection` for details.
	#
	# FIXME: Cannot be used if RTA is enabled. So `niti` or `--erasure` only.
	fun combinations(r: Int): Collection[SequenceRead[E]]
	do
		var res = new CombinationCollection[E](self, r)
		res.are_sorted = true
		res.are_unique = true
		return res
	end