Please tell me how to fix this: module mystery$\_$tb$()$;

reg $x1,s,x2$;

wire $f$;

mystery check$(x1,x2,s,f)$;

initial begin

$x1=0$;$s=0$;$x2=0$;

#$10$

$x1=0$;$s=0$;$x2=1$

#$10$

$x1=0$;$s=1$;$x2=0$;

#$10$

$x1=0$;$s=1$;$x2=1$

#$10$

$x1=1$;$s=0$;$x2=0$;

#$10$

$x1=1$;$s=0$;$x2=1$;

#$10$

$x1=1$;$s=1$;$x2=0$

#$10$

$x1=1$;$s=1$;$x2=1$;

$stop;

end

endmodule

module mystery $($f$,$ x$1,$ s$,$ x$2)$ :

input $x1,s,x2$;

output $f$;

wire $a,b$;

assign $a=x1,|s$;

assign $b=x2$&$s$;

assign $f=a$&$b$;

endmodule