/// <summary>
/// Changes sign of current input.
/// </summary>
private void ChangeSign(object sender, RoutedEventArgs e)
{
if (operationExecutor.State == ExecutorState.ResultCalculated ||
operationExecutor.State == ExecutorState.Error)
{
inputValidator.ClearInput();
SetSecondaryOutput("");
operationExecutor.Clear();
}
SetMainOutput(inputValidator.ChangeSign());
}