Combine ``columns`` into a single string.

    Example:
        >>> combine_columns(['eape', 'xml'])
        'example'

    Args:
        columns (iterable): ordered columns to combine

    Returns:
        String of combined columns

def combine_columns(columns):
    """Combine ``columns`` into a single string.

    Example:
        >>> combine_columns(['eape', 'xml'])
        'example'

    Args:
        columns (iterable): ordered columns to combine

    Returns:
        String of combined columns
    """
    columns_zipped = itertools.zip_longest(*columns)
    return ''.join(x for zipped in columns_zipped for x in zipped if x)