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greg is a re-entrant peg/leg, with some bug fixes. <http://piumarta.com/software/peg/> the most comprehensive example of greg usage is in nagaqueen, an ooc grammar, used in rock, an ooc compiler written in ooc. <http://github.com/nddrylliog/nagaqueen> <http://github.com/nddrylliog/rock> peg/leg is copyright (c) 2007 by Ian Piumarta released under an MIT license. as is greg. This fork of greg supports the memoization of failed rule application attempts. For a language with more elaborated syntax and operators, a PEG parser can spend exponentially long time to try the same rule application at the same input location. The typical solution for this problem is to assume that a grammar rule always returns the same result (or fail) at the same location and remember the value (or failure), effectively in the 2D table of (location, rule_id) -> value. An interesting feature that Ian's leg has is that it defers the semantic action execution until the system "commits" a successfull parse. (That is why putting printf() in a failed choice in a rule does not show any debug log, BTW.) Because of this, we cannot remember the successful result; as it is calculated later while attempting a lot of reduandant failures. So, instead, this fork only remembers the fact that the rule failed; next time it trys the same rule at the location, it short-cuts the work and fails.