# Pastebin P4iBthnt
    follow [] = []
    follow (Knot op os:xs) = head' : tails xs hp
      where
        hp = fwd dir op
        head' = Knot hp (Set.insert hp os)

        tails = para (\case
                         Nil -> const []
                         Cons k@(Knot p s) (sns, r) ->
                           \hp' ->
                             if touching hp' p then k : sns
                             else let np = towardsPrev p hp' in Knot np (Set.insert np s) : r np)