module as01d1 (s, co, b, add, a, ci);
   input b, add, a, ci;
   output s, co;
`ifdef SYNTH
    reg is, ico;
    buf (s,is);
    buf (co,ico);
    always @(a or b or add or ci)
       case (add)
          1'b0: {ico,is} = a - b - ~ci; // subtract - c should be high
          1'b1: {ico,is} = a + b + ci; // add
       endcase
`else
   nand                 G2(N2, N16, N17);
   or                   G5(co, N6, N8);
   and                  G6(N6, a, N13),
                        G8(N8, ci, N12);
   nand                 G9(N9, add, b),
                        G10(N10, N24, N25);
   or                   G12(N12, a, N2);
   nand                 G13(N13, N26, N9);
   or                   G16(N16, add, b);
   nand                 G17(N17, add, b);
   or                   G18(N18, N23, ci);
   nand                 G19(N19, N23, ci);
   and                  G20(s, N18, N19);
   or                   G21(N21, a, N10);
   nand                 G22(N22, a, N10);
   and                  G23(N23, N21, N22);
   or                   G24(N24, add, b);
   nand                 G25(N25, add, b);
   or                   G26(N26, add, b);
`endif
endmodule