diff --git a/dune/tectonic/myconvexproblem.hh b/dune/tectonic/myconvexproblem.hh
index c2eb3445fc29da16aa2eb3daf19dfee96abb1648..76542cd4b1f2875d06ab7de2796271827f8cf25c 100644
--- a/dune/tectonic/myconvexproblem.hh
+++ b/dune/tectonic/myconvexproblem.hh
@@ -27,6 +27,14 @@ class MyConvexProblem {
                   VectorType const &f)
       : A(A), phi(phi), f(f) {};
 
+  /* Just for debugging */
+  double operator()(VectorType const &x) const {
+    double ret = phi(x) - (f * x);
+    VectorType tmp(x.size());
+    A.mv(x, tmp);
+    return ret + 0.5 * (tmp * x);
+  }
+
   MatrixType const &A;
   Dune::GlobalNonlinearity<MatrixType, VectorType> const &phi;