*  
    addsub4.v
    4bit Adder-Subtractor
 */
module addsub4(a, b, cin, s, cout);
  input  [3:0] a, b;
  input        cin;
  output [3:0] s;
  output       cout;

  assign {cout, s} = cin? a - b: a + b;
endmodule