In our scheme, a back-off state (or function) q b produces estimates that are target structures t 2 ∈ T ∆ with score s ∈ R, given some information of the source tree fragment t 1 ∈ T Σ .
a function q b : In our QA application, we only use the leaves of the input subtree t 1 and use lexicons or entity/predicate linkers to retrieve KB entities, KB relations or a compound of a disambiguating relation and an entity from backoff states ent, pred and bridge, respectively.