FSDs have been cited as a component of GPSG which cannot be captured declaratively. We show that the apparent dynamic aspect of these defaults is a consequence of the difficulty of capturing them within the formal framework in which GPSG was originally defined. The monadic second-order quantification of L2KP allows FSDs to be defined in a much more direct way, clarifying their nature significantly.
ECPO, despite being a fundamental principle of GPSG, has been ignored by existing model-theoretic treatments of GPSG. We argue that this is a consequence of the fact that these treatments focus on axiomatizing the set of trees licensed by a given GPSG grammar, while ECPO, being a universal principle of natural language, is a characteristic of the class of sets of trees that analyze natural languages. That is, it is not a property of trees, but is, rather, a property of sets of trees. L2KP, again principally because it supports second-order quantification, allows us to capture, in a limited way, universal principles like ECPO.
Finally, we note that, while L2KP is powerful enough to capture principles like FSDs and ECPO in a natural way, its formal expressive power is well-defined. The class of finite trees that are definable in L2KP are a very slight generalization of the class of sets of trees definable in the phrase-structure formalism underlying GPSG.