@Override public void visitLiteralExpression(@NotNull PsiLiteralExpression expression) {
    startElement(expression);

    DfType dfType = DfaPsiUtil.fromLiteral(expression);
    push(dfType, expression);
    if (dfType == DfTypes.NULL) {
      addNullCheck(expression);
    }

    finishElement(expression);
  }
@Override public void visitLiteralExpression(@NotNull PsiLiteralExpression expression)