The transformation between T t and T3 should be one of the simplest (accordingly to the operations cost; see section 3.3), and then the search would be limited to the retrieval of a pair (T1,T2) sharing an equivalent transformation.
this is still time-consuming, and we are trying to define a general way to limit the search in such a tree structure space, for example based on tree indexing for efficiency (Daelemans et al., 1997).