Are phonosyntactic rules necessary?

Selkirk (1986) proposes that rules of the phrasal phonology apply to a structure called P-structure that is derived from, but not isomorphic to, syntactic structure. While Selkirk claims that most rules of external sandhi fall into the category of P-structure rules, there are three rules that she suggests may still require direct reference to syntactic structure. These are Mende Consonant Mutation (Conteh et al. 1986), Kimatuumbi Vowel Shortening (Odden this volume), and ‘perhaps even rules like English wanna contraction’ (Selkirk 1986: 374). Selkirk says that these rules ‘appear to be sensitive to a richer array of syntactic conditions than would be expressible via the translation of syntax into P-structure’.