This is an example of an overconstrained scheduling problem. It uses a special problem fact (NullTool) instead of None to differentiate when it is impossible to arrive at a solution. The xml solver file is not strictly required but is representative of a more complex use case. It works fine in 8.23.0a0, but not in 8.28+
python -m venv venv . venv/bin/activate pip install -r requirements.txt python main.py