code: func IsQuadNotExist(err error) bool {
  	if err == ErrQuadNotExist {
  		return true
  	}
  	de, ok := err.(*DeltaError)
  	return ok && de.Err == ErrQuadNotExist
  }
docs: // IsQuadNotExist returns whether an error is a DeltaError
  // with the Err field equal to ErrQuadNotExist.