Add hint to library class.

git-svn-id: https://svn.dealii.org/trunk@3194 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/examples/step-9/step-9.cc b/deal.II/examples/step-9/step-9.cc
index 5ef997513a8599767a9a0c6195044e6bd1e70e1d..dc169920c8cdbb613396c81921dbba712e7b8570 100644 (file)
--- a/deal.II/examples/step-9/step-9.cc
+++ b/deal.II/examples/step-9/step-9.cc
@@ -493,7 +493,17 @@ BoundaryValues<dim>::value_list (const vector<Point<dim> > &points,
                                 // approximation of the gradient on
                                 // each cell and weighs that with a
                                 // power of the mesh size, as
-                                // described in the introduction. The
+                                // described in the introduction.
+                                // This class is a simple version of
+                                // the ``DerivativeApproximation''
+                                // class in the library, that uses
+                                // similar techniques to obtain
+                                // finite difference approximations
+                                // of the gradient of a finite
+                                // element field, or if higher
+                                // derivatives.
+                                //
+                                // The
                                 // class has one public static
                                 // function ``estimate'' that is
                                 // called to compute a vector of