tcfail218.hs:16:5:
    Overlapping instances for C [a] Bool arising from a use of ‘foo’
    Matching instances:
      instance C [a] b -- Defined at tcfail218.hs:8:29
      instance C [Int] Bool -- Defined at tcfail218.hs:7:29
    (The choice depends on the instantiation of ‘a’
     To pick the first instance above, use IncoherentInstances
     when compiling the other instance declarations)
    In the expression: foo
    In an equation for ‘x’: x = foo