Function undo_step 

pub fn undo_step(
    doc: &mut AutoCommit,
    step: &RunStep,
    by: Option<&str>,
    authors: &AuthorDirectory,
) -> StepOutcome

Undo one application’s share of a run against the document that app owns.

The transaction is left open; the owning app commits, so the undo lands under the person who asked for it and replicates like any other edit.