Specify the exact lookahead to be used by the subrule.
Valid values are true and false. Default is true. Normally symbols are matched greedily so that if a symbol can be matched immediately or by exiting the subrule the parser chooses to match the symbol immediately.
Rule-specific version of the backtrack grammar option. See Grammar options for details.
Rule-specific version of the memoize grammar option. See Grammar options for details.