package fourpack is
	type u_x01z is ('X','0','1','Z');		-- u_x01z is unresolved
	type u_x01z_vector is array (natural range <>) of u_x01z;
	function resolve4 (s:u_x01z_vector) return u_x01z;
	subtype x01z is resolve4 u_x01z;
	-- x01z is a resolved subtype which uses the resolution function resolve4
	type x01z_vector is array (natural range <>) of x01z;
end fourpack;

package body fourpack is
	type x01z_table is array (u_x01z,u_x01z) of u_x01z;
	constant resolution_table : x01z_table := (
		('X','X','X','X'),
		('X','0','X','0'),
		('X','X','1','1'),
		('X','0','1','Z'));
	function resolve4 (s:u_x01z_vector) return u_x01z is
		variable result : u_x01z := 'Z';
	begin
		if (s'length = 1) then
			return s(s'low);
		else
			for i in s'range loop
				result := resolution_table(result,s(i));
			end loop;
		end if;
		return result;
	end resolve4;
end fourpack;