In this paper we investigate the factors conditioning a morphological alternation on verbal heads in Lai. We show that this alternation eludes a simple characterization and instead exhibits a many-to-many form-function mapping. We will further show that the facts can be given a straightforward analysis in terms of default conditions based on valence and polarity, together with various constructional overrides. Our analysis thus follows recent proposals in HPSG, in particular Malouf (forthcoming), in using a constructional type hierarchy with defaults (cooperating constructions) as an alternative to an Optimality Theoretic system of ranked violable constraints.