module mi21d2 (zn, i0, i1, s);
   input i0, i1, s;
   output zn;
   and                  G2(N2, s, i1),
                        G3(N3, N4, i0);
   not                  G4(N4, s);
   nor                  G6(zn, N2, N3);
endmodule