Each node pair has a form Ψ s , Ψ t , A L , where Ψ s ∈ {Ω s , L} n represents the source-side node of length n. It consists of a sequence of lexical words in the source-side vocabulary Ω s and nonterminals L. Ψ t ∈ {Ω t , L} m similarly represents the target-side node of length m. A L preserves correspondences between the non-terminals in the source and target nodes.
we extract a pair of leaf nodes as a pair of lexical strings.