redef fun []=(index, item)
	do
		assert index >= 0 and index <= _length
		if written then reset
		if index == _length then
			add item
			return
		end
		var it = _items
		var ip = it.char_to_byte_index(index)
		var c = it.char_at(ip)
		var clen = c.u8char_len
		var itemlen = item.u8char_len
		var size_diff = itemlen - clen
		if size_diff > 0 then
			rshift_bytes(ip + clen, size_diff)
		else if size_diff < 0 then
			lshift_bytes(ip + clen, -size_diff)
		end
		_byte_length += size_diff
		it.set_char_at(ip, item)
	end