Function plan_revert 

pub fn plan_revert(
    doc: &mut AutoCommit,
    run: &ChangeRun,
    by: Option<&str>,
    authors: &dyn ResolveAuthor,
) -> Result<RevertOutcome, RevertError>

Work out what undoing run would do, without touching the document.

The dry run behind an affordance that says before the user commits to it what stays: “undo, except the title Bob renamed since”.

Errors

Propagates RevertError from run_bounds when run cannot be addressed in doc.