48. Safe MUX

How do you plan to solve it?

module safe_mux2 (
    input  [7:0] a,
    input  [7:0] b,
    input        sel,
    output reg [7:0] y,
    output reg       sel_unknown
);
    always @* 
    begin
        y           = 8'h00;
        sel_unknown = 1'b0;

        if (sel === 1'b0) 
        begin
            y <= a;
        end
        else if (sel === 1'b1) 
        begin
            y <= b;
        end
        // `sel` is either `x` or `z`
        else 
        begin
            sel_unknown <= 1'b1;
            y <= 8'h00;
        end
    end
endmodule