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