I want to modify a XPATH parser, but I don’t know how to start with.
The soure code look like:

Any help would be appreciated:)
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This looks like a “shift reduce” (see the S and Rs) table for an LALR parser. My guess is that it is produced by the GOLD parser which produces application-independent parse tables.
But you won’t reasonably be able to modify this without the original grammmar, and the parser generator.
Why would you want to modify a perfectly working XPath parser anyway? If it isn’t perfect, why don’t you just use a perfect one?