In an MSCFG, the elementary structures are rewrite rules containing not a source and target, but an arbitrary number M of strings where X is the head of the rule and η m is a string of terminal and non-terminal symbols.
4 In this paper, for notational convenience, we will use a specialized version of Equation 3 in which we define a single γ as the source side string, and α 1 , ...α N as an arbitrary number N of target side strings: at each derivation step, one non-terminal in γ is chosen and all the nonterminals with same indices in α 1 , ..., α N will be rewritten using a single rule.