@Nullable
  private static Boolean evaluateBoolean(@NotNull PyExpression expression) {
    if (expression instanceof PyBoolLiteralExpression) {
      if (PyNames.DEBUG.equals(expression.getText())) {
        return null;
      }
      return ((PyBoolLiteralExpression)expression).getValue();
    }
    else if (expression instanceof PyReferenceExpression && LanguageLevel.forElement(expression).isPython2()) {
      final String text = ((PyQualifiedExpression)expression).getReferencedName();
      if (PyNames.TRUE.equals(text)) {
        return true;
      }
      if (PyNames.FALSE.equals(text)) {
        return false;
      }
    }

    return null;
  }
@Nullable
  private static Boolean evaluateBoolean(@NotNull PyExpression expression)