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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 get_format_extension(fmt):
    '''
    Returns the recommended extension for a given format
    '''

    if fmt is None:
        return 'dict'

    fmt = fmt.lower()
    if fmt not in _converter_map:
        raise RuntimeError('Unknown basis set format "{}"'.format(fmt))

    return _converter_map[fmt]['extension']