Whether one wants the full expressivity of modally controlled structural rules is a matter of debate.  Vermaat (2006) takes the strong position that the cross-linguistic possibilities for displacement can be fully captured in terms of the right branch extraction postulates discussed above, together with their mirror images for extraction from left branches.  Under this view, which is consistent with the radical lexicalism of the original categorial systems, this postulate set is fixed at the level of Universal Grammar, and variation is reduced to the language-specific lexica that determine which of the available extraction patterns are activated.