In an n-SCFG, the elementary structures are rewrite rules of n − 1 target sides: where X is a non-terminal symbol, γ 1 is the source side string of terminal and non-terminal symbols, and γ 2 , ...γ n are the target side strings.
at each derivation step, one non-terminal in γ 1 is chosen and all the corresponding non-terminals with the same index in {γ 2 , ..., γ n } are rewritten using a single rule.