T7438.hs:6:14: error:
    Couldn't match expected type ‘t1’ with actual type ‘t’
      ‘t’ is untouchable
        inside the constraints: t2 ~ t3
        bound by a pattern with constructor:
                   Nil :: forall (k :: BOX) (b :: k). Thrist b b,
                 in an equation for ‘go’
        at T7438.hs:6:4-6
    ‘t’ is a rigid type variable bound by
        the inferred type of go :: Thrist t2 t3 -> t -> t1 at T7438.hs:6:1
    ‘t1’ is a rigid type variable bound by
         the inferred type of go :: Thrist t2 t3 -> t -> t1 at T7438.hs:6:1
    Possible fix: add a type signature for ‘go’
    In the expression: acc
    In an equation for ‘go’: go Nil acc = acc
    Relevant bindings include
      acc :: t (bound at T7438.hs:6:8)
      go :: Thrist t2 t3 -> t -> t1 (bound at T7438.hs:6:1)