'Transform the following node tree::
<node>
<title>
<section>
<title>
into ::
<node>
<title>
<subtitle>'

def promote_subtitle(self, node):

    (subsection, index) = self.candidate_index(node)
    if (index is None):
        return None
    subtitle = nodes.subtitle()
    subtitle.attributes.update(subsection.attributes)
    subtitle[:] = subsection[0][:]
    node[:] = (((node[:1] + [subtitle]) + node[1:index]) + subsection[1:])
    return 1
