[…] it is natural to base the operation on nondistinctness rather than identity, because the features unspecified in underlying structure (such as number, in predicate position) also make no independent contribution to sentence interpretation, being added by what are, in essence, redundancy rules, and are, in fact, simply a reflection of context. Thus they are recoverable in the sense that the context that determined them is still present in string after deletion of the item in question.
- Chomsky (1969), a pag.182