69. Binary Subtractor

How do you plan to solve it?

module sub4_2c(
    input  [3:0] a,
    input  [3:0] b,
    output [3:0] diff,
    output       bout
);


wire [4:0] sum = a + {1'b0, ~b} + 1'b1;

assign diff = sum[3:0];
assign bout = ~sum[4]; // borrow = NOT carry-out

endmodule