Hierarchical vertex split hierarchical extension of the vertex split. Hierarchical vertex splits are the building blocks of hierarchical surfaces. They are derived from #GtsSplit objects but add pointers to their parents in the vertex split tree and information on the state (collapsed or expanded) of their children. Together with #GtsEHeapPair fields these informations allow to manage any valid sequence of expansions or collapses. Casts @klass to #GtsHSplitClass. @klass: a descendant of #GtsHSplitClass. Casts @obj to #GtsHSplit. @obj: a descendant of #GtsHSplit. Evaluates to %TRUE if @obj is a #GtsHSplit, %FALSE otherwise. @obj: a pointer to test. The hierarchical split vertex class derived from #GtsSplitClass. @parent_class: The hierarchical split vertex object. @split: The parent object. @index: Used when managing priority heaps for vertex split and collapse. @parent: Parent of this hierarchical split vertex in the split vertex tree. @nchild: Number of expanded children of this vertex split (0, 1, or 2). @Returns: @klass: @vs: @Returns: @hs: @hsurface: @hs: @hsurface: @hs: @hsurface: