code: public final boolean errorsSince(Checkpoint checkpoint) {
      Checkpoint impl = checkpoint;
      if (impl.owner != this) {
        throw new IllegalArgumentException(
            "Can only call errorsSince on a Checkpoint instance that was returned from this same "
                + "reporter");
      }
      return getCurrentNumberOfErrors() > impl.errorsSoFar;
    }
docs: Returns true iff errors have occurred since {@code checkpoint} was obtained from {@link
  #checkpoint}.
  
  <p>Useful for callers whose outputs are dependent on whether some code path resulted in new
  errors (for example, returning an error node if parsing encountered errors).