Checking the validity of std_logic_vector value @testbench

    function has_X( value : std_logic_vector) return boolean is

    begin

        for index in value'range loop

            if value( index) = 'X' then

                return true;

            end if;

        end loop;

        return false;

    end function has_X;

    FUNCTION Is_X ( s : std_logic_vector  ) RETURN  BOOLEAN IS

    BEGIN

        FOR i IN s'RANGE LOOP

            CASE s(i) IS

                WHEN 'U' | 'X' | 'Z' | 'W' | '-' => RETURN TRUE;

                WHEN OTHERS => NULL;

            END CASE;

        END LOOP;

        RETURN FALSE;

    END;