def join_nonempty(l):
    """
    Join all of the nonempty string with a plus sign.
    
    >>> join_nonempty(('x1 + x2 + x1:x2', 'x3 + x4'))
    'x1 + x2 + x1:x2 + x3 + x4'
    >>> join_nonempty(('abc', '', '123', ''))
    'abc + 123'
    """
    return ' + '.join(s for s in l if s != '')