Zwicky [Zwicky, A. M., 1985, How to describe inflection, Berkeley Linguistic Society 11.372-86, p. 372] argues that an adequate realizational theory of inflection must incorporate two different kinds of realization rules: "First there are rules of EXPONENCE, describing how certain combinations of morphosyntactic features are realized, in the context of certain other bundles, as morphological operations. The following principle of English is a typical rule of exponence: In the context of [CAT: verb], [VFORM: past] is realized by the suffixation of /d/. Then there are rules of REFERRAL, stipulating that certain combinations of features have the same realization as certain others. The following principle of English is a typical rule of referral: In the context of [CAT: verb], [VFORM: pastprt] has the same realization as [VFORM: past]. All realization rules are treated as expressing DEFAULTS, which are automatically overidden by more specific rules (and these in turn by still more specific rules, and so on).
- Stump (2004), a pag.94-95