code: func Is(e error, original error) bool {
  
  	if e == original {
  		return true
  	}
  
  	if e, ok := e.(*Error); ok {
  		return Is(e.Err, original)
  	}
  
  	if original, ok := original.(*Error); ok {
  		return Is(e, original.Err)
  	}
  
  	return false
  }
docs: // Is detects whether the error is equal to a given error. Errors
  // are considered equal by this function if they are the same object,
  // or if they both contain the same error inside an errors.Error.