def concat_filter_strings(filter_strings, operator='&'):
    """
    Helper function to combine ami filter strings

    Parameters
    ----------
    filter_strings: ``list``
        The valid filter strings to combine

    operator: ``str``
        The operator to place between the filter strings. This can either be
        ``&`` or ``|``, for ``and`` or ``or`` respectively.
    """
    if len(filter_strings) == 0:
        raise ValueError('filter_strings must have at least one element')
    elif len(filter_strings) == 1:
        return filter_strings[0]
    else:
        sep = ')' + operator + '('
        return '(' + sep.join(filter_strings) + ')'