module scbtnb (co, q, qn, ci, cp, sdn, cdn);
   input ci, cp, sdn, cdn;
   output co, q, qn;
   reg iq, iqn, ico;

   buf (q,iq);
   buf (qn,iqn);
   and (co,ci,ico);
   always @(posedge cp)
      if (ci)
         begin
            {ico,iqn} = iqn+1;
            iq = ~iqn;
         end
   always @(sdn or cdn)
      case ({cdn,sdn})
         2'b00: assign {ico,iq,iqn} = 3'b110;
         2'b01: assign {ico,iq,iqn} = 3'b001;
         2'b10: assign {ico,iq,iqn} = 3'b110;
         2'b11: deassign {ico,iq,iqn} ;
      endcase

endmodule