There is a wrong output in the 7th example in the description. For the input "111111222" the output should be "9,[1[12]]" as in the tests.

I am concerned about a test case: ssstntntsssstsntntssssttntntsssstssntntsssststntntsssststntttssstssntttsssttntttssstsntttssstnttttnsstnsstnsstnsstnssnnn with an input 12345. If I am right that we have to treat numbers from the input separately (i.e. '1', '2', ...), after most of the commands we have tnsstnsstnsstnsstnssnnn to do with a stack like this ['5', '4', '3', '2', '1']. "tnss" means "Pop a value off the stack and output it as a character" and by the previous test cases I assume that command expects a unicode number (eg. tnss with '65' on top of the stack gives 'A') but the output is expected '12345'. Is it an issue and test should contain "tnst" instead of "tnss" parts or I've made a mistake somewhere?

That's why they use the term interval, not a set, because it is not a set of integers (for which we can calculate a number of elements) but an interval of real numbers on the number line (which has a length). It get clearer when you get to the examples: interval [1, 5] has a length of 4 and a set {1, 2, 3, 4, 5} would have 5 elements.