* It is recommended that a functions using the flags states so in its
* documentation.
*
+ * There is another set of user data, namely a #void *#, for each line, quad,
+ * etc. Just like the user flags, this field is not available for vertices,
+ * which does no harm since the vertices have a unique and continuous number
+ * unlike the structured objects lines and quads. You can access these user
+ * pointers through the functions #user_pointer()#, #clear_user_pointer()#
+ * and #set_user_pointer(p)# in the accessor classes.
+ *
+ * You should not rely on any specific value for the user pointer if you
+ * haven't set it yourself before. In special, the pointers are not inherited
+ * upon refinement. In principle, all user pointers should be #NULL# pointers
+ * at the time a line, quad, etc comes into life. #NULL# pointers are always
+ * a good choice, since they raise an exception when being dereferenced.
+ *
*
* \subsection{Boundary approximation}
*