gistfile1.txt

multi trait_mod:<is>(Attribute $attr, :$notted!) {
    my $acc-name = $attr.name.substr(2);
    $attr.package.HOW.add_method: $attr.package, 'not-' ~ $acc-name, method () {
        not self."$acc-name"()
    }
}

class A {
    has $.x is notted;
}

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