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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 rationalize_until(until=None):
 """ Rationalizes the `until` argument used by other functions. This function accepts datetime and timedelta instances as well as integers representing epoch values. """

    if until is None:
        until = "+inf"
    elif isinstance(until, datetime):
        until = to_unix(until)
    elif isinstance(until, timedelta):
        until = to_unix((datetime.utcnow() + until))
    return until