The tree specification would work better if a tree could be expressed as a collections of vertices instead of a collection of nodes.

[MathieuDuponchelle]

I have implemented serialization of the class hierarchy in g-ir-doc-tool, and the resulting tree for a class with multiple inheritance (interfaces) is full of redundancy, for example :

        <tree>
          <item>
            <link href="https://developer.gnome.org/gobject/unstable//gobject-The-Base-Object-Type.html#GObject">GObject.Object</link>
            <item>
              <link href="https://developer.gnome.org/gobject/unstable//gobject-The-Base-Object-Type.html#GInitiallyUnowned">GObject.InitiallyUnowned</link>
              <item>
                <link xref="GES.Object">GES.Object</link>
                <item>
                  <link xref="GES.Source">GES.Source</link>
                  <item>
                    <link xref="GES.UriSource">GES.UriSource</link>
                  </item>
                </item>
              </item>
            </item>
          </item>
          <item>
            <link xref="index">GObject.GInterface</link>
            <item>
              <link href="http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html//GstChildProxy.html">Gst.ChildProxy</link>
              <item>
                <link xref="GES.Object">GES.Object</link>
                <item>
                  <link xref="GES.Source">GES.Source</link>
                  <item>
                    <link xref="GES.UriSource">GES.UriSource</link>
                  </item>
                </item>
              </item>
            </item>
            <item>
              <link xref="GES.Playable">GES.Playable</link>
              <item>
                <link xref="GES.Source">GES.Source</link>
                <item>
                  <link xref="GES.UriSource">GES.UriSource</link>
                </item>
              </item>
            </item>
          </item>
        </tree>

as each branch from the ancestor "leaves" to the child has to be fully specified.

If defined in terms of vertices, with "implicit nodes", the structure would be much cleaner / easier to parse:

<vertex from=GInterface to=GstChildProxy />
<vertex from=GInterface to=GES.Playable />
<vertex from=GObject to=GInitiallyUnowned />
  <vertex from=GstChildProxy to=GES.Object />
  <vertex from=GInitiallyUnowned to=GES.Object />
     <vertex from=GES.Playable to=GES.Source />
     <vertex from=GES.Object to=GES.Source />
       <vertex from=GES.Source to=GES.UriSource />