  The modal approach represents a “higher level of nonmonotonic involvement” than default logic. The unprovability construct is represented explicitly in the language, by means of a modal operator L informally interpreted as ‘provable’ (or, as  in  McDermott &amp; Doyle 1980,  by the dual of this operator).[19]  Although McDermott and Doyle’s terminology is different from Reiter’s, the logical ideas are very similar—the essence of their approach, like Reiter’s, is a fixpoint definition of the extensions of a nonmonotonic logic. Incorporating nonmonoticity in the object language creates some additional complexities, which in the early modal approach show up mainly in proliferation of the logics and difficulties in evaluating the merits of the alternatives. As better foundations for the modal approach emerged, it became possible to prove the expected theorems concerning equivalence of modal formalisms with default  logic.[20]