@staticmethod
def _get_subfield_names():
    """Return list of column headers. The first several are required. The
        last dozen or so are unlikely to be found on the RTM. This is because
        the user is allowed as many Design Output Solutions as they need."""
    field_names = ['Procedure Step', 'Need', 'Design Input']
    for i in range(1, 20):
        field_names.append(f'Solution Level {str(i)}')
    return field_names
