Stateful programs often become complex beasts as they grow. Program state incohesively spread across a bunch of variables, spuriously guarded by even more variables, is what I refer to as implicit state. When working with such code, we have to reconstruct a model mentally, identifying possible states and transitions between them, to modify the program with confidence. Even if a test suite can help, the process is tedious and error-prone, and I insist we should have our tools do the heavy lift...
