def copy(self, old_name, new_name, index=None):
        """
        Copies an old sheet with the old_name to a new sheet with new_name.

        If an optional index argument is not provided then the created
        sheet is appended at the end. Returns the new sheet.
        """
        if index is None:
            index = len(self)
        self._copy(old_name, new_name, index)
        return self[new_name]
Copies an old sheet with the old_name to a new sheet with new_name.

        If an optional index argument is not provided then the created
        sheet is appended at the end. Returns the new sheet.