tcfail040.hs:19:5: error:
    Ambiguous type variable ‘a0’ arising from a use of ‘<<’
    prevents the constraint ‘(ORD a0)’ from being solved.
    Probable fix: use a type annotation to specify what ‘a0’ should be.
    These potential instance exist:
      instance ORD (a -> b) -- Defined at tcfail040.hs:17:10
    In the first argument of ‘(===)’, namely ‘(<<)’
    In the expression: (<<) === (<<)
    In an equation for ‘f’: f = (<<) === (<<)