Thanks a lot. It took a while since I came back to this question. It worked brilliantly comfortable and with great ease. But ... It produced another incomprehensible message:

PacletInstall::compat: The paclet CodeInspector was successfully installed, but it will not be available in this session because it is not compatible with the currently running version of the Wolfram system.

I did obey the recommendation from github.com to build it with my version of Mathematica.

Same messages with PacletInstall and path as with cmake --install.

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The philosophy is that it is easy to convert from Concrete -> Aggregate syntax, and also from Aggregate -> Abstract. So at those 3 stages, we do scans for rules that are defined.

For people playing at home: Concrete Syntax: parse tree with whitespace are comments preserved (think InputForm) Aggregate Syntax: all whitespace and comments have been stripped. Abstract Syntax: nodes are transformed into more abstract forms (think FullForm)

Most rules will be for Abstract syntax.

You can ask yourself: Does the difference between 1+2 and Plus[1,2] matter for my rule? If no, then use Abstract Syntax. If yes, then ask: would whitespace affect the rule? If no, then use Aggregate Syntax. Otherwise use Concrete Syntax.

It should be noted: I am also thinking of the best way to represent a level lower than Concrete Syntax, working directly with tokens. This would be where a rule for ;; at the end of a line would be. It would be wrong to have the rule ";; at the end of a line" as Concrete Syntax, because it is hard to check "end of line" in a tree structure. But with a list of tokens, just need to check ;; token is next to \n token. Maybe it will be called TokenRules or something.

No explicit list right now. It is a goal to make the errors available programmatically, for better filtering and also easier translation to other languages, etc.