1050.lean:7:2-18:5: error: invalid mutual definition, result types must be in the same universe level, resulting type for `Foo.bar` is
  Foo as bs → bs : Type (u + 1)
and for `Foo.bar_aux` is
  bs : Type u
1050.lean:24:8-24:12: warning: declaration uses 'sorry'
1050.lean:24:8-24:12: warning: declaration uses 'sorry'