We adopt the synchronous tree substitution grammar (STSG) formalism (Eisner, 2003) which can model non-isomorphic tree structures (the grammar rules can comprise trees of arbitrary depth) and is thus suited to text-rewriting tasks which typically involve a number of local modifications to the input text.
we propose quasi-synchronous tree substitution grammar (QTSG) as a flexible formalism to learn general treeedits from loosely-aligned phrase structure trees.