for my $x (0,
           "0",
           0.0,
           dualvar(0,1),
           dualvar(0,"0 "),
            '----------------',
            1,
            "foo",
            "00",
           "0 ",
           undef,
           "",
           dualvar(1,0),
           "0.0",
          ) {
    print "$x before: ",is_zero($x),"\n";
    my $y = ($x+1.1).$x;
    print "$x after:  ",is_zero($x),"\n";
}