Both trees in the tree set participate in the derivation: The auxiliary tree β:for ex1 adjoins at the root of T2, whereas the auxiliary tree β:for ex2 adjoins at the root of the higher discourse unit.
since we saw from example (66a) that the interpretation of this higher discourse unit is explanation(interp(T2),interp(T1)), the interpretation associated with the adjoined β:for ex2 node both embeds and abstracts this interpretation, yielding John's never returning what he borrows is one instance of a set of explanations.