T5236.hs:13:9: error:
    Couldn't match type ‘B’ with ‘A’
      arising from a functional dependency between:
        constraint ‘Id A B’
          arising from the type signature for:
                         loop :: Id A B => Bool
        instance ‘Id B B’ at T5236.hs:11:10-15
    Inaccessible code in
      the type signature for:
        loop :: Id A B => Bool
    In the ambiguity check for ‘loop’
    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
    In the type signature: loop :: Id A B => Bool