# Pastebin j6tRqunb
follow :: Dir -> [Knot] -> [Knot]
follow dir = para (\case
                  Nil -> []
                  Cons (Knot hp hs) (_, []) -> let np = fwd dir hp in [Knot np (Set.insert np hs)]
                  Cons ex@(Knot p s) (_, done@(Knot hp _:_))
                    | touching hp p -> ex:done
                    | otherwise -> let np = towardsHead p hp in
                                         Knot np (Set.insert np s) : done)