# Where is the `key` in `_item`?
	# return -1 if not found
	private fun index(key: K): Int
	do
		var l = _last_index
		if l < _items.length and _items[l].first == key then return l

		var i = 0
		while i < _items.length do
			if _items[i].first == key then
				_last_index = i
				return i
			end
			i += 1
		end
		return -1
	end