fd-loop.hs:12:10:
    Variable occurs more often in a constraint than in the instance head
      in the constraint: C a b
    (Use -XUndecidableInstances to permit this)
    In the instance declaration for `Eq (T a)'

fd-loop.hs:12:10:
    Variable occurs more often in a constraint than in the instance head
      in the constraint: Eq b
    (Use -XUndecidableInstances to permit this)
    In the instance declaration for `Eq (T a)'