* after creation of the triangulation. It follows that active
* cells on level 1 may not be coarsenend.
*
+ * The main use of this flag is to ensure that each cell has at least one
+ * neighbor in each coordinate direction (i.e. each cell has at least a
+ * left or right, and at least an upper or lower neighbor in 2d). This is
+ * a necessary precondition for some algorihms that compute finite
+ * differences between cells. The DerivativeApproximation class is one of
+ * these algorithms that require that a triangulation is coarsest_level_1
+ * unless all cells already have at least one neighbor in each coordinate
+ * direction on the coarsest level.
+ *
* <li> @p smoothing_on_refinement:
* This flag sums up all smoothing algorithms which may be performed upon
* refinement by flagging some more cells for refinement.