Roles6.hs:5:1:
    Role mismatch on variable a:
      Annotation says nominal but role representational is required
    In the data declaration for ‛Foo’

Roles6.hs:5:1:
    Role mismatch on variable b:
      Annotation says representational but role nominal is required
    In the data declaration for ‛Foo’

Roles6.hs:7:1:
    Wrong number of roles listed in role annotation;
    Expected 2, got 3:
      type role Foo nominal representational phantom