# Returns the real index for a modulo index.
	#
	# ~~~
	# var a = [10,20,30]
	# assert a.modulo_index(1) == 1
	# assert a.modulo_index(3) == 0
	# assert a.modulo_index(-1) == 2
	# assert a.modulo_index(-10) == 2
	# ~~~
	#
	# REQUIRE `not_empty`
	fun modulo_index(index: Int): Int
	do
		var length = self.length
		if index >= 0 then
			return index % length
		else
			return length - (-1 - index) % length - 1
		end
	end