For this purpose, we introduce the null dependency structure e. For any operation OP and dependency structure X, Y, we have OP(X, Y) = e if this operation is not defined in Definition 5 OP(X, e) = X OP(e, X) = X Because part of a hypothesis may have a null dependency structure, we cannot calculate dependency LM scores on some of the related words.
we give a discount for each of these words.