# Jump to the next item `step` times.
	#
	# ~~~
	# var i = [11, 22, 33, 44].iterator
	# assert i.item == 11
	# i.next_by 2
	# assert i.item == 33
	# ~~~
	#
	# `next_by` should be used instead of looping on `next` because is takes care
	# of stopping if the end of iteration is reached prematurely whereas a loop of
	# `next` will abort because of the precondition on `is_ok`.
	#
	# ~~~
	# i.next_by 100
	# assert not i.is_ok
	# ~~~
	#
	# If `step` is negative, this method aborts.
	# But specific subclasses can change this and do something more meaningful instead.
	#
	# Require `is_ok`
	fun next_by(step: Int)
	do
		assert step >= 0
		while is_ok and step > 0 do
			next
			step -= 1
		end
	end