explain calling of compress() in step-40.

diff --git a/examples/step-40/step-40.cc b/examples/step-40/step-40.cc
index e03278ce6575199e55c9442b02b51d8e389da4cf..1d57447e9c424edc88a42e6471417acb43f47367 100644 (file)
--- a/examples/step-40/step-40.cc
+++ b/examples/step-40/step-40.cc
@@ -419,6 +419,11 @@ namespace Step40
                                                   system_rhs);
         }
 
+    // Notice that the assembling above is just a local operation. So, to
+    // fomulate the "global" linear system, a synchronization between all
+    // processors is needed. This could be done by invoke the function
+    // compress(). See @ref GlossCompress  "Compressing distributed objects"
+    // for more information on what is compress() designed to do.
     system_matrix.compress (VectorOperation::add);
     system_rhs.compress (VectorOperation::add);
   }