Tried the example in the README, but it failed and only gave an empty solution when writing the constraint as string ("a*2==b"). The same example with a lambda constraint worked fine.

Further investigation reveals that, after the getSolutions() call, variable a just loses its domain, that is problem._variables gives a=[].

Affected versions: 2.3.0 and 2.3.1 from PyPI.
I downgraded to version 2.2.3 and there the string constraint example worked, too.

Note: you should really write tests for the code in the README, too, to ensure it works.