# It's actually a lot more complex than this, but just
# pretend it's this simple...
#
sub HashRef () {   ### empty prototype
  return Moose::Meta::TypeConstraint->lookup('HashRef');
}

##</code><code>##

    isa => HashRef

##</code><code>##

package A;
has data => (is => 'ro');
package B;
has data => (is => 'rw');