As you can see, this just selects all elements at particular tree level.
So, my foldMapS2 looks similar to foldMap from Foldable, but i can't figure out, how should i define instances of Foldable (and Monoid?) to achieve the same functionality?
You cannot. Foldable is not general enough; it does not allow you to define folds which can observe the *structure* of the container being folded over (such as the level in a tree, the number of children of a given node, etc.). It corresponds to simply "flattening" the structure into a list of elements, turning each of them into a value of some monoid, and then applying mconcat. However, you should be able to define a general fold for Tape, with type foldTape :: (a -> [b] -> b) -> Tape a -> b and then define foldMapS2 in terms of foldTape. -Brent