Cross word puzzle solve using python constraint library

[Larz60+]

The documentation for problem.addVariable always shows items in list as numerics, never strings, although I don't see any constraints on doing so:

Output:
addVariable(self, variable, domain)

Add a variable to the problem
Example:

>>> problem = Problem()
>>> problem.addVariable("a", [1, 2])
>>> problem.getSolution() in ({'a': 1}, {'a': 2})
True

    Parameters:
        variable - Object representing a problem variable
                   (type=hashable object) 
        domain - Set of items defining the possible values that the given variable may assume
                   (type=list, tuple, or instance of Domain) 

addVariables(self, variables, domain)

Add one or more variables to the problem
Example:

>>> problem = Problem()
>>> problem.addVariables(["a", "b"], [1, 2, 3])
>>> solutions = problem.getSolutions()
>>> len(solutions)
9

>>> {'a': 3, 'b': 1} in solutions
True

    Parameters:
        variables - Any object containing a sequence of objects represeting problem variables
                   (type=sequence of hashable objects) 
        domain - Set of items defining the possible values that the given variables may assume
                   (type=list, tuple, or instance of Domain) 

If that were an issue, I would expect it to be documented, but don't see it.

I found the example of building a crossword which uses constraints: http://www.cromulentrambling.com/2017/01...ython.html
Please note that the author builds problem thusly:

problem.addVariables(list(range(s*s)), [0,1])

again with a list of numerics for domain.