For the type of control where a structural transformation has to be blocked, we can turn to extraction islands: phrases that do not allow a gap, such as adjuncts. Compare ‘Mary fell asleep during the concert’, with the adjunct phrase ‘during the concert’ of type s\s, with the ill-formed ‘I know what Mary fell asleep during —’. To make the adjunct inaccessible, one can assign ‘during’ the type (□(s\s))/np: for the □ to be eliminated, the whole adjunct phrase has to be enclosed by a structural ◊, which then acts as a barrier for extraction. Without such a barrier, the extraction postulates as formulated above would allow the ill-formed sentence to be derived.  This strategy of projecting island barriers from lexical type assignments was introduced by Morrill; see the discussion of ‘structural inhibition’ in Morrill 1994.