mod71.hs:4:9: error:
    Found hole: _ :: t1
    Where: ‘t1’ is a rigid type variable bound by
                the inferred type of f :: Num a => (t1 -> a -> t) -> t
                at mod71.hs:4:1
    Relevant bindings include
      x :: t1 -> a -> t (bound at mod71.hs:4:3)
      f :: (t1 -> a -> t) -> t (bound at mod71.hs:4:1)
    In the first argument of ‘x’, namely ‘_’
    In the expression: x _ 1
    In an equation for ‘f’: f x = x _ 1