Hi Damien, I reply to the Moose list because this might be interesting to other people too. I guess you are referring to the extension methods in the package 'PetitAnalyzer'? The methods #matches:, #matchesIn:. #matchingRangesIn:, ... are part of the core package 'PetitParser' and are well commented (I think). The methods in the package 'PetitAnalyzer' are called match*, because this is not an equality operation. They do not only support the comparison of two parsers, but can also compare patterns with parser instances (essentially this is a little Prolog engine, very similar to the refactoring engine). The matching and rewriting of parsers is explained in my PhD (http://scg.unibe.ch/archive/phd/renggli-phd.pdf) in Section "6.2.5 Declarative Grammar Rewriting". For example: " matches a sequence of any two parsers that are the same " any := PPPattern any. pattern := any , any. pattern asParser match: $a asParser , $b inContext: Dictionary new. " --> false, because $a and $b are different " pattern asParser match: $a asParser , $a asParser inContext: Dictionary new. " --> true, because $a and $a are the same " If the match is successful, the patterns are bound to the matching parsers. In the example above the dictionary would contain an entry: any -> $a asParser There are many tests in PPSearcherTest. Fancy patterns you can also see in PPRewriterTest and PPOptimizer. This looks correct to me. This is to recurse into the graph of parsers. PPParser is a superclass of PPUnresolvedParser, therefore you can send #def: to any instance of PPUnresolvedParser. PPUnresolvedParser and #def: are nice to quickly hack something ugly together, better not (over)use them ... Lukas -- Lukas Renggli www.lukas-renggli.ch