Test: 31 and 32. Finally, refactored these two cases into the test named "two digits convert to two words"
TPP: expression -> function (20 -> NUMBER_TO_ONE_WORD.floorKey(number)).
Before this transformation happens, I actually refactor the NUMBER_TO_ONE_WORD map to a NavigableMap.
On the other hand, if only add the case 31, you can see a unconditional -> if transformation, which is finally not existing any more after the TPP happens. This reminders me that the actual TPP may not be that obvious in the first place. And, sometimes unconditional -> if transformation could be "temporary transformation" in such case.