Adding a new feature

New methods and functions should be fully type hinted (where possible) and have docstrings. Adhere to the Coding style guidelines for code and docstrings.

If adding a new feature, you should add new unit tests and these should fully cover your patch.

Note: All tests live under Cogent3/tests with test directory structure and test module names largely mirroring those under Cogent3/src/cogent3.