Strongly agree. Even more, it's also a gread heuristic to use if you want to refactor existing code into something more testable. At times, the simple process

Strongly agree. Even more, it's also a gread heuristic to use if you want to refactor existing code into something more testable. At times, the simple process of figuring out what the really necessary points of mutation/IO are and how to "fence them off" is all I need to simplify big chunks of an existing "ball of mud".