THML Tag to process the sync element. A sync tag is always empty and
immediately follows what it marks. With types of Strong's and morph these are
to become w elements that surround the word that they modify. This requires
that we find the last text element and surround it with a w element. If the
last text element is already surrounded with a w element then this is added
to it. As a simplifying assumption, we will assume that the text element is
not contained by anything except perhaps by a w element.