message: Make elem_mat mutable
diff: @@ -355,7 +355,7 @@ class SumIntegrator : public BilinearFormIntegrator
  {
  private:
  int own_integrators;
  - DenseMatrix elem_mat;
  + mutable DenseMatrix elem_mat;
  Array<BilinearFormIntegrator*> integrators;
  public:
  