Function undo_summary 

pub fn undo_summary(
    conversation: &mut Conversation,
    store: &dyn Store,
    log: &str,
    by: Author,
) -> Result<Uncompacted, UndoError>

Undoes the newest compaction of conversation, the conversation saved in the log log of store: the entries its summary replaced, read from that log as they were just before it, are put back in the summary’s place with their ids (Conversation::restore by by), and its Compacted aside goes. Entries added after the summary stay after them. Undone one after another, compactions come back newest first, each with the summary it built on.

Errors

UndoError, changing nothing.