We are proposing the following convention to guarantee recoverability of deletion: a deletion operation can eliminate only a dummy element, or a formative explicitly mentioned in the structure index (for example, 'you' in imperatives), or the designated representative of a category (for example, the 'wh'-question transformations that delete Noun Phrases are in fact limited to indefinite Pronouns [...]) or an element that is otherwise represented in the sentence in a fixed position.
- Chomsky (1969), a pag.144-145