The `but` keyword takes an instance and something to mix into it.

So you can say something like this:

    0 but True

---

Since `True` is a `Bool` it automatically creates a role that has a method `Bool` that returns `True`.

    my $role = role { method Bool { True }}

You can do that manually if you want.

    0 but role { method Bool { True }}

---

It creates a new subclass of Int and mixes in the role.

So:

    0 but True

Is roughly equivalent to:

    do {
      my constant but-true = anon role { method Bool { True }}
      my $class = anon class :: is Int does but-true {}

      $class.new( 0 )
    }

I think the you mean: `subset Minor of Person where *.age < 18`