Some other semantic parser learning systems which need MRL in the form of Prolog (Tang and Mooney, 2001) or λ-calculus (Zettlemoyer and Collins, 2007;Wong and Mooney, 2007) do not use productions of the MRG but instead use predicates of the MRL.
this operator will not eliminate other occurrences of t 1 and t 2 on the RHS of other productions in which they do not occur adjacent to each other.