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