message: Comment on 2 * BONDED_ERAS.
diff: @@ -465,7 +465,8 @@ where
  let era = Era::new(highway, start_height);
  let _ = self.active_eras.insert(era_id, era);
  - // Remove the era that has become obsolete now.
  + // Remove the era that has become obsolete now. We keep 2 * BONDED_ERAS past eras because
  + // the oldest bonded era could still receive blocks that refer to BONDED_ERAS before that.
  if let Some(obsolete_era_id) = era_id.checked_sub(2 * BONDED_ERAS + 1) {
  self.active_eras.remove(&obsolete_era_id);
  }
  