[…] a theory that I will call "Theory I". In this theory we assume that sentences have the S-structure postulated in the preceding discussion, including trace and PRO. Consider now two variants of Theory I. One variant, call it Ia, assumes as above that Move-α forms these S-structures from base-generated D-structures. Theories Ia and Ib share certain interpretive rules, specifically, the rules of construal relating antecedents to overt anaphors and to certain extraposed phrases [...] and the rules of control determining the referential possibilities for PRO [...] Theories Ia and Ib differ, however, in their treatment of antecedent-trace relations [...] In Theory Ia, these relations are established in the syntax by the rule Move-α; in Theory Ib, by new interpretive rules of the LF-component satisfying [...] whatever [...] conditions may distinguish trace from PRO.
- Chomsky (1993), Pag. 90