def header(string, flank=True):
    """

    Generate a Header String

    :param string: a string.
    :type string: ``str``
    :param flank: if True, flank the header with line breaks.
    :type flank: ``bool``
    :return:
    """
    sep_line = '-' * len(string)
    if flank:
        print('\n')
    print('\n{0}\n{1}\n{2}\n'.format(sep_line, string, sep_line))
    if flank:
        print('\n')
