Once I figured out a key concept, how and when to use Str and Add, I managed to pass it in C# without much trouble.
The fact the description mentions "Exp", though in some occasions more specifically requires "Str" doesn't help :s
However, the "core" example test shows how to use these classes. Let's say you have "abcd" (a sequence of 4 x normal). This should be treated as a fold of Add with the seed being the first list item wrapped with Str: Add(Add(Add(Str(a),b),c),d).

sorting is O(n * LOG(n))