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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 format_datetime(d: PotentialDatetimeType, fmt: str, default: str = None) -> Optional[str]: """ Format a datetime with a ``strftime`` format specification string, or return ``default`` if the input is ``None``. """

    d = coerce_to_pendulum(d)
    if d is None:
        return default
    return d.strftime(fmt)