Document "maybe" operator for covspectrum advanced queries

[corneliusroemer]

Erik Boehm had a question that would be perfectly answered by covSpectrum's maybe query:

I was wondering if you had any insight on why the % of the S:371 mutation is so "low" (91-3%) in the World and European data, while it is >99% in the Swiss data

However I couldn't find any documentation on how to use the maybe operator, even what it is. But I very well remember talking to @chaoran-chen about this operator and found it in the code:
https://github.com/GenSpectrum/LAPIS-SILO/blob/164ccd0571d0987f007c412b429fe12935a95b91/include/silo/query_engine/filter_expressions/maybe.h#L21
https://github.com/GenSpectrum/LAPIS/blob/0c6e7ad46569e37a90c3a0f6018355585cdca791/server/src/main/java/ch/ethz/lapis/api/query/Maybe.java#L9

Is maybe just syntactic sugar for !(S:371.)?

https://cov-spectrum.org/explore/World/AllSamples/Past6M/variants?variantQuery=%21%28S%3A371.%29&

[chaoran-chen]

You're right, we have quite a bit of a documentation backlog at the moment. We'll try to catch up after LAPIS v2. At the moment, the only mention of it is in the LAPIS paper. There is a "“Maybe” queries" section: https://bmcbioinformatics.biomedcentral.com/articles/10.1186/s12859-023-05364-3#Sec2

Do I understand right that you are interested in the number of sequences that potentially have a mutation at S:371? Then, !S:371. indeed does exactly that. Maybe is most useful if you would like to know the sequences that maybe have a very specific nucleotide mutation because nucleotides have all the different ambiguity codes like M, R, etc.