/// <summary>
 /// Removes one character from the right of current input.
 /// </summary>
 private void RemoveCharacter(object sender, RoutedEventArgs e)
 {
     if (operationExecutor.State == ExecutorState.ResultCalculated ||
         operationExecutor.State == ExecutorState.Error)
     {
         ClearAll(sender, e);
         return;
     }
     SetMainOutput(inputValidator.ClearCharacter());
 }