tcfail040.hs:19:14:
    Ambiguous type variable `a0' in the constraint:
      (ORD a0) arising from a use of `<<'
    Probable fix: add a type signature that fixes these type variable(s)
    In the second argument of `(===)', namely `(<<)'
    In the expression: (<<) === (<<)
    In an equation for `f': f = (<<) === (<<)