module scctnb (co, q, qn, ci, cp, cdn);
   input ci, cp, cdn;
   output co, q, qn;

   reg iqn, ico;
   not (q,iqn);
   buf (qn,iqn);
   buf (co,ico);
   always @(posedge cp)
      if (ci)
           {ico,iqn} = iqn+1;
   always @(cdn)
      case ({cdn})
         1'b0: assign {ico,iqn} = 2'b01;
         1'b1: deassign {ico,iqn} ;
      endcase

endmodule