 Obligations can be violated, and impermissible things do  happen.[15]  However, as researchers turned to generalizations of alethic modal logic, they began to consider wider classes of modal logics, including ones where the necessity operator was not truth-implicating. This too encouraged seeing deontic logic as falling within modal logic so-generalized. In fact, recognizing possibilities like this helped to fuel the generalizations of what began with a focus on alethic modal logic to normal modal logics (Lemmon &amp; Scott 1977).