'Add an existing worksheet (at an optional index).'

def _add_sheet(self, worksheet, index=None):

    cls = Worksheet
    if self.write_only:
        cls = WriteOnlyWorksheet
    if (not isinstance(worksheet, cls)):
        raise TypeError(("The    parameter    you    have    given    is    not    of    the    type    '%s'" % cls.__name__))
    if (worksheet.parent != self):
        raise ValueError('You    cannot    add    worksheets    from    another    workbook.')
    if (index is None):
        self.worksheets.append(worksheet)
    else:
        self.worksheets.insert(index, worksheet)
