Add another assertion

dune/tectonic/myblockproblem.hh

@@ -187,6 +187,25 @@ template <class MyConvexProblemTypeTEMPLATE> class MyBlockProblem {
     // -grad is needed for Newton step
     linearization.b *= -1.0;
 
+    // b should be a descent direction
+    {
+      VectorType const direction = linearization.b;
+      VectorType tmp = linearization.b;      //  b
+      linearization.A.mmv(u, tmp);           //  b-Au
+      double const localA = tmp * direction; // <b-Au,v>
+
+      linearization.A.mv(direction, tmp);    //  Av
+      double const localb = tmp * direction; // <Av,v>
+
+      MyDirectionalConvexFunction<Dune::GlobalNonlinearity<block_size>> const
+      psi(localA, localb, problem.phi, u, direction);
+
+      Interval<double> D;
+      psi.subDiff(0, D);
+      if (!isnan(D[1]))
+        assert(D[1] <= 0);
+    }
+
     // apply truncation to system
     for (size_t row = 0; row < linearization.A.N(); ++row) {
       auto const end = linearization.A[row].end();