Is it hard to add sheets to an existing excel file on the disk?
is it hard to add sheets to an existing excel file on the disk?

```
import pandas as pd 
import numpy as np
a=pd.DataFrame(np.random.random((3,1)))
excel_writer=pd.ExcelWriter('c:\excel.xlsx')
a.to_excel(excel_writer, 'a1')
excel_writer.save()

excel_writer=pd.ExcelWriter('c:\excel.xlsx')
a.to_excel(excel_writer, 'a2')
excel_writer.save()
```

here only sheet 'a2" is save, but I like to save both 'a1' and 'a2'.

I know it is possible to add sheets to an existing workbook.
