def to_series(items, conjunction='and'):
    """
    Formats the given items as a series string.
    
    Example:
    
    >>> to_series([1, 2, 3])
    '1, 2 and 3'
    
    :param items: the items to format in a series
    :param conjunction: the series conjunction
    :return: the items series
    :rtype: str
    """
    if not items:
        return ''
    prefix = ', '.join([str(i) for i in items[:-1]])
    suffix = str(items[-1])
    if not prefix:
        return suffix
    else:
        return (' ' + conjunction + ' ').join([prefix, suffix])