If we merge [5b] and [6b], which is the sentence DRS corresponding with [6a], we obtain [6c]. In this representation, the discourse referent z does not have access to y, because y is introduced in a DRS that is not accessible to the DRS in which z is introduced, and therefore it is not possible to bind z to y.  In other words, if [6a] is preceded by [5a], the pronoun cannot be anaphorically linked to the indefinite. This prediction appears to be correct.