fn cursor(&mut self) -> (Self::Cursor, Self::Storage) {
        if let Some(cursor) = self.cursor_through(Antichain::new().borrow()) {
            cursor
        }
        else {
            panic!("unable to acquire complete cursor for trace; is it closed?");
        }
    }
Provides a cursor over updates contained in the trace.
differential-dataflow/src/trace/mod.rs