T1900.hs:13:10:
    Could not deduce (Depend s0 ~ Depend s)
    from the context (Bug s)
      bound by the type signature for check :: Bug s => Depend s -> Bool
      at T1900.hs:13:10-36
    NB: ‘Depend’ is a type function, and may not be injective
    The type variable ‘s0’ is ambiguous
    Expected type: Depend s -> Bool
      Actual type: Depend s0 -> Bool
    In the ambiguity check for: forall s. Bug s => Depend s -> Bool
    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
    In the type signature for ‘check’:
      check :: Bug s => Depend s -> Bool