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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_arrow(value, format_string):
 """Format an arrow datetime object. :param value: The arrow datetime object. :param format_string: The date format string :returns: Returns a string representation of the given arrow datetime object, formatted according to the given format string. .. note:: Do not use this filter to format date/times presented to an end user. Instead use ``datetimeformat`` or ``dateformat`` from Invenio-I18N. """

    assert isinstance(value, arrow.Arrow)
    return value.format(format_string)