* vertex with the same number of the old quad.
*
*
+ * \subsection{Implementational conventions for three spatial dimensions}
+ *
+ * By convention, we will use the following numbering for vertices, lines and
+ * faces of hexehedra in three space dimensions. Before giving these
+ * conventions we declare the following sketch to be the standard way of
+ * drawing 3d pictures of hexhedra:
+ * \begin{verbatim}
+ * *-------* *-------*
+ * /| | / /|
+ * / | | / / |
+ * / | | / / |
+ * * | | *-------* |
+ * | *-------* | | *
+ * | / / | | /
+ * | / / | | /
+ * |/ / | |/
+ * *-------* *-------*
+ * \end{verbatim}
+ * The left part of the picture shows the left, bottom and back face of the
+ * cube, while the right one shall be the top, right and front face. You may
+ * recover the whole cube by moving the two parts together into one.
+ *
+ * \subsubsection{Vertices}
+ *
+ * The vertices on the front face are numbered exactly the same way as are
+ * the vertices on a quadrilateral. The vertices on the back face are numbered
+ * similarly by moving the front face to the back (no turning, no twisting,
+ * just a shift):
+ * \begin{verbatim}
+ * 7-------6 7-------6
+ * /| | / /|
+ * / | | / / |
+ * / | | / / |
+ * 3 | | 3-------2 |
+ * | 4-------5 | | 5
+ * | / / | | /
+ * | / / | | /
+ * |/ / | |/
+ * 0-------1 0-------1
+ * \end{verbatim}
+ *
+ * \subsubsection{Lines}
+ *
+ * Here, the same holds as for the vertices: the lines of the front face are
+ * numbered as for the quadrilateral, for the back face they are just shifted.
+ * Finally, the four lines connecting front and back face are numbered:
+ * \begin{verbatim}
+ * *---6---* *-------*
+ * /| | / /|
+ * 11 | 5 11 10 |
+ * / 7 | / / |
+ * * | | *---2---* |
+ * | *---4---* | | *
+ * | / / | 1 /
+ * 3 8 9 3 | 9
+ * |/ / | |/
+ * *---0---* *---0---*
+ * \end{verbatim}
+ * The direction of the front and back lines is as for the respective faces, while
+ * the connecting faces always point to the back:
+ * \begin{verbatim}
+ * *--->---* *--->---*
+ * /| | / /|
+ * ^ | ^ ^ ^ ^
+ * / ^ | / / |
+ * * | | *--->---* |
+ * | *--->---* | | *
+ * | / / | ^ /
+ * ^ ^ ^ ^ | ^
+ * |/ / | |/
+ * *--->---* *--->---*
+ * \end{verbatim}
+ *
+ * \subsubsection{Faces}
+ *
+ * The faces are numbered in the same order as the lines were numbered: front
+ * face, back face, then the four side faces:
+ * \begin{verbatim}
+ * *-------* *-------*
+ * /| | / /|
+ * / | 2 | / 5 / |
+ * / | | / / |
+ * * | | *-------* |
+ * | 6 *-------* | | 4 *
+ * | / / | | /
+ * | / 3 / | 1 | /
+ * |/ / | |/
+ * *-------* *-------*
+ * \end{verbatim}
+ *
+ *
* \subsection{Warning}
*
* It seems impossible to preserve #const#ness of a triangulation through