for loops

a <= '0'
b <= '0'
c <= '0'
d <= '0'
e <= '0'

for ai in 0 to 1 loop
    for bi in 0 to 1 loop
        for ci in 0 to 1 loop
            for di in 0 to 1 loop
                for ei in 0 to 1 loop
                    wait for 1 ns;
                    e <= not e;
                end loop;
                d <= not d;
            end loop;
            c <= not c;
        end loop;
        b <= not b;
    end loop;
    a <= not a;
end loop;