case sel is
when “000” => y<= data(0);
when “001” => y<= data(1);
when “010” => y<= data(2);
when “011” => y<= data(3);
1) when others => y<= ‘0’;
2) when others => y<= ‘Z’;
3) when others => y<= ‘X’;
4) when others => NULL;
end case;
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