# The index of the first occurrence of `item`, starting from pos.
	# Return -1 if `item` is not found.
	# Comparison is done with `==`.
	#
	#     var a = [10,20,30,10,20,30]
	#     assert a.index_of_from(20, 3)   == 4
	#     assert a.index_of_from(20, 4)   == 4
	#     assert a.index_of_from(20, 5)   == -1
	fun index_of_from(item: nullable Object, pos: Int): Int
	do
		var p = 0
		var i = iterator
		while i.is_ok do
			if p>=pos and i.item == item then return i.index
			i.next
			p += 1
		end
		return -1
	end