
<interactive>:5:10:
    Illegal instance declaration for `C [Int]'
      (All instance types must be of the form (T a1 ... an)
       where a1 ... an are *distinct type variables*,
       and each type variable appears at most once in the instance head.
       Use -XFlexibleInstances if you want to disable this.)
    In the instance declaration for `C [Int]'

<interactive>:8:10:
    Overlapping instance declarations:
      instance C a => C [a] -- Defined at <interactive>:8:10
      instance C [Int] -- Defined at <interactive>:7:10