gistfile1.txt

multi trait_mod:<is>(Method $meth, :$notted!) {
    my $name = $meth.name;
    $meth.package.HOW.add_method: $meth.package, 'not-' ~ $name, method () {
        not self."$name"()
    }
}

class A {
    method x() is notted {
        True
    }
}

my $o = A.new();
say $o.x;
say $o.not-x;