perl 6 trait problem

gistfile1.p6

role SomeTrait {
  method apply_handles($attr: Mu $pkg) {
    my $name = $attr.name;
    my $accessor = $name.subst(/^../, '');
    $pkg.HOW.find_method($pkg, $accessor).wrap(-> $obj, |args {
      say "$name accessor";
      callsame;
    });
  }
}

multi trait_mod:<is>(Attribute $var, :$wtf!) {
  $var.set_rw();
  $var does SomeTrait;
}

class foo {
  has $.x is wtf;
}

my $foo = foo.new;
$foo.x = 'bar';