Added
- Added a Changelog
Changed
- Error messages on `logics.classes.propositional.proof_theories.natural_deduction`.
`NaturalDeductionSystem`'s `is_correct_derivation` method now returns a list of tuples
as `error_list`, where the first member is the step index (`int`) and the second is an
`str` with the error, instead of a list of `str`.