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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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]