For a given SCFG production l → α f , α c , we ask, given the source RHS α f that is assumed given (e.g., provided by a syntactic parser), which of its RHS elements are also present in α c .
we write: where k h , k i l , k j r ('k' for keep) are binary variables that are true if and only if constituents H, L i , R j (respectively) of the source RHS α f are present in the target side α c .