Function entry_message 

pub fn entry_message(
    doc: &AutoCommit,
    actor: &ActorId,
    context: &ActionContext,
) -> Option<String>

The message context is written with by actor on doc: as the continuation of the actor’s previous entry when the change continues its gesture (a head of doc is the actor’s, of the same gesture and step), else whole. A function of the document, so a replaying device writes the same message, hence the same change.