tcfail047.hs:7:10: error:
    • Illegal instance declaration for ‘A (a, (b, c))’
        (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 FlexibleInstances if you want to disable this.)
    • In the instance declaration for ‘A (a, (b, c))’