# Return the index-th element but wrap
	#
	# Whereas `self[]` requires the index to exists, the `modulo` accessor automatically
	# wraps overbound and underbouds indexes.
	#
	# ~~~
	# var a = [10,20,30]
	# assert a.modulo(1) == 20
	# assert a.modulo(3) == 10
	# assert a.modulo(-1) == 30
	# assert a.modulo(-10) == 30
	# ~~~
	#
	# REQUIRE `not_empty`
	# ENSURE `result == self[modulo_index(index)]`
	fun modulo(index: Int): E do return self[modulo_index(index)]