QST: to_excel() deletes the existed sheets
- As per Pandas [documentation](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.to_excel.html), the to_excel() deletes the existed sheets in the excel document using the code below. I am not sure what is the issue.
```python
with pd.ExcelWriter('pay_past_5_years.xlsx', mode='a') as writer:  
    df.to_excel(writer, sheet_name='Sheet_name_3')
```
- The second question the mode is give error when using small 'a' char in mode. Here tip was found in [stackoverflow](https://stackoverflow.com/questions/54863238/pandas-excelwriter-valueerror-append-mode-is-not-supported-with-xlsxwriter/55683301)
```python
ValueError: Append mode is not supported with xlsxwriter!
````
**The Pandas version is 1.0.5**

Thank you
