code: func isDup(a, b *Delta) *Delta {
  	if out := isDeletionDup(a, b); out != nil {
  		return out
  	}
  	// TODO: Detect other duplicate situations? Are there any?
  	return nil
  }
docs: // If a & b represent the same event, returns the delta that ought to be kept.
  // Otherwise, returns nil.
  // TODO: is there anything other than deletions that need deduping?