'Promote (replace) the content.data with the first attribute
of the current content.data that is a I{list}.  Note: the
content.data may be empty or contain only _x attributes.
In either case, the content.data is assigned an empty list.
@param content: An array content.
@type content: L{Content}'

def promote(self, content):

    for (n, v) in content.data:
        if isinstance(v, list):
            content.data = v
            return
    content.data = []
