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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 _return_wrapper(fits, return_all, start, trace):
 """If the user wants to get all of the models back, this will return a list of the ARIMA models, otherwise it will just return the model. If this is called from the end of the function, ``fits`` will already be a list. We *know* that if a function call makes it here, ``fits`` is NOT None or it would have thrown an exception in :func:`_post_ppc_arima`. Parameters fits : iterable or ARIMA The ARIMA(s) return_all : bool Whether to return all. """

    # make sure it's an iterable
    if not is_iterable(fits):
        fits = [fits]

    # whether to print the final runtime
    if trace:
        print('Total fit time: %.3f seconds' % (time.time() - start))

    # which to return? if not all, then first index (assume sorted)
    if not return_all:
        return fits[0]
    return fits