Suppose that the derivation has reached the stage Σ, which we may take to be a set {SO1,...,SOn} of syntactic objects. One of the operations of Chl is a procedure that selects a lexical item LI from the numeration, reducing its index by 1, and introduces it into the derivation as SOn+1. Call the operation "Select". - Chomsky (1995), a pag.226