fix(normalize): Add Xor simplification to simplify_boolean

[aryanjsingh]

Summary

This PR adds support for simplifying Xor expressions within the simplify_boolean transformation. This prevents premature decomposition of Xor with constant values, which could lead to errors in later transformations like linearize as reported in the issue.

Changes

• cpmpy/transformations/normalize.py: Added simplification logic for the 'xor' operator in simplify_boolean transformation. This is done by adding a new case in the visit_operator method of the SimplifyVisitor class. The logic correctly handles constant BoolVal arguments, simplifying the expression before it can be decomposed incorrectly by later transformations.
• tests/test_trans_simplify.py: Added a new test method test_xor_simplify to verify the new simplification logic for the Xor operator. This test includes the specific case from the issue, as well as other edge cases to ensure correctness.

Related Issue

Closes #747

[tias]

Hi aryan,

Thanks for submitting a PR.

I'm not sure why you closed it?

It does look like you are introducing a whole lot of code. I've looked a bit closer into the bug, and there are 2 things:

1. Xor decomp can create a reified Const + Const == 1 constraint
2. linearize does not handle Const + Const == 1 well

The 2nd one is the less obvious one that needs attention, but even if it would be fixed, the 1st would still be a needless waste of effort.

So instead of adding xor to simplification, perhaps we should add simplification before the xor decomposition...