T1897b.hs:16:1:
    Could not deduce (Depend a0 ~ Depend a)
    from the context (Bug a)
      bound by the inferred type for ‛isValid’:
                 Bug a => [Depend a] -> Bool
      at T1897b.hs:16:1-41
    NB: ‛Depend’ is a type function, and may not be injective
    The type variable ‛a0’ is ambiguous
    Expected type: [Depend a] -> Bool
      Actual type: [Depend a0] -> Bool
    When checking that ‛isValid’
      has the inferred type ‛forall a. Bug a => [Depend a] -> Bool’
    Probable cause: the inferred type is ambiguous