Add reaction grammar

[julia-sprenger]

A reaction grammar implementation exists already in x4i, but as pyparser implementation here: https://github.com/brown170/x4i/blob/main/src/x4i/exfor_grammars.py

[gschnabel]

Thanks for locating the grammar in the x4i package. I think we should acknowledge Dave for his idea to use a formal grammar some 15-20 years ago. Maybe at the end of the readme file could be good place.

Having said that, I would prefer to start from scratch for the development of the grammar, using the EXFOR manual as the only guiding document. It will help us to gain some experience with the grammar modeling, which probably will be helpful later in this project (or any potential follow-up project) when it becomes technically more complicated.

What do you think about this perspective?

[julia-sprenger]

I am not sure if the EXFOR manual will be sufficient as guiding document, as in some places there's still room for interpretation. In contrast to the manual, an already existing implementation for EXFOR aspects is more precise.

[gschnabel]

That's true. We could also use existing EXFOR entries and base our development from the very start on comprehensive tests that iterate over the entire library. Another reason why I am a bit hesitant to directly pick code from Dave's repo is the license, GPL. We would be forced to apply the GPL to our developments as well and I think this license is quite problematic in interconnected systems of software. The LGPL has later been created for more legal clarity, but it is unfortunately not the license of x4i. In general, I would rather prefer to develop our projects using permissive licenses rather than copyleft ones.

[julia-sprenger]

The test approach is definitely needed.
You are right with the licence. Maybe it's better to only use it as inspiration and don't start porting from there. Alternatively this issue could also result in a solution: brown170/x4i#42