Principe de sous-ensemble
Elsewhere Condition (Anderson 1986:4):
[W]henever one rule is more specific than another in the sense that the forms subject to the first constitute a proper subset of those subject to the second, the application of the more specific rule precludes the later application of the more general, less specific one. As Anderson points out, the Elsewhere Condition has two important consequences for the applicability of rules of inflectional morphology: (i) Rules that specify the realization of some set of [inflectional] features [...] prevent the later application of other rules whose structural descriptions refer to a proper subset of those features. (ii) Stems that are lexically characterized for some set of features block the operation of rules specifying a (non-null) subset of those same features.
Anderson observes that double plural nouns in Breton appear to counterexemplify the second consequence of the Elsewhere Condition (ii), but he argues that the conflict is merely apparent because he postulates that roots are collective.