tcfail121.hs:13:9:
    Overlapping instances for Foo [a]
      arising from a use of `op'
    Matching instances:
      instance [overlap ok] Foo a => Foo [a]
        -- Defined at tcfail121.hs:9:10-25
      instance [overlap ok] Foo [Int] -- Defined at tcfail121.hs:10:10-18
    (The choice depends on the instantiation of `a'
     To pick the first instance above, use -XIncoherentInstances
     when compiling the other instance declarations)
    In the expression: op x
    In an equation for `foo': foo x = op x