T2247.hs:7:10: error:
    • Illegal instance declaration for ‘FD a b’
        The liberal coverage condition fails in class ‘FD’
          for functional dependency: ‘a -> b’
        Reason: lhs type ‘a’ does not determine rhs type ‘b’
        Un-determined variable: b
    • In the instance declaration for ‘FD a b’