# Pastebin rJL2Gsbu
MacBook-Pro-de-Fernando:roles fernando$ cat Foo.pm6 
multi trait_mod:<is>(Routine $r, :&command!) is export {
    $r.wrap: my method (|) {
        my \rest = callwith self;
        if ?rest {
            self.save: command rest
        }
        rest
    }
}

role Foo {
    method save (|) { }
}
MacBook-Pro-de-Fernando:roles fernando$ cat z1.p6 
class Bar does Foo {
    method foo is command{.uc.flip.say} {
        'meow'
    }
}
Bar.foo: 42
MacBook-Pro-de-Fernando:roles fernando$ perl6 -I. -MFoo z1.p6
WOEM
MacBook-Pro-de-Fernando:roles fernando$ cat z2.p6 
use Bar;
Bar.foo: 42
MacBook-Pro-de-Fernando:roles fernando$ cat Bar.pm6 
use Foo;
class Bar does Foo {
    method foo is command{.uc.flip.say} {
        'meow'
    }
}
MacBook-Pro-de-Fernando:roles fernando$ perl6 -I. -MFoo z2.p6
Cannot invoke this object (REPR: Null; VMNull)
  in method <anon> at /Users/fernando/Tests/roles/Foo.pm6 (Foo) line 3
  in block <unit> at z2.p6 line 2