code: func InvariantErrorf(format string, a ...interface{}) error {
  	var (
  		invariantFormat = InvariantViolatedMetricName + ": " + format
  		err             = fmt.Errorf(invariantFormat, a...)
  	)
  
  	panicIfEnvSetWithMessage(err.Error())
  	return err
  }
docs: // InvariantErrorf constructs a new error, prefixed with a string indicating that an invariant
  // violation occurred. Optionally panics if the ShouldPanicEnvironmentVariableName is set to "true".