Some small improvements.

dune/fem-dg/solver/dg.hh

@@ -41,6 +41,7 @@ namespace Fem
              class DiffusionModel,
              class Additional>
 #ifdef EULER_WRAPPER_TEST
+#error
   class DGOperator : public DuneODE :: OdeSolverInterface< DestinationImp >
 #else
   class DGOperator : public Fem::SpaceOperatorInterface< DestinationImp >

dune/fem-dg/solver/rungekuttasolver.hh

@@ -123,6 +123,7 @@ namespace Fem
     typedef ImplicitOperator  ImplicitOperatorType;
 
     typedef DuneODE :: OdeSolverInterface< typename Operator :: DestinationType > BaseType;
+    typedef RungeKuttaSolver< Operator, ExplicitOperator, ImplicitOperator, LinearInverseOperator > ThisType;
 
     template <class AdvOp, class DiffOp>
     struct IsSame
@@ -358,10 +359,10 @@ namespace Fem
       }
     }
 
-    void operator () (const DestinationType& U, DestinationType& dest )
+    void operator () (const DestinationType& U, DestinationType& dest ) const
     {
       dest.assign( U );
-      solve( dest );
+      const_cast< ThisType& > (*this).solve( dest );
     }
 
     //! solver the ODE

python/dune/femdg/_operators.py

@@ -478,6 +478,8 @@ def createRungeKuttaSolver( space, fullOperator, explOperator=None, implOperator
     #                          ['"op"_a', '"explOp"_a', '"implOp"_a',
     #                           'pybind11::keep_alive< 1, 2 >()', 'pybind11::keep_alive< 1, 3 >()','pybind11::keep_alive< 1, 4 >()' ])
 
+    solve = Method('solve', '''[]( DuneType &self, typename DuneType::DestinationType &u) { self.solve(u); }''' );
+
     #solve = Method('step', '&DuneType::solve', extra=['"target"_a'])
 
-    return load(includes, typeName, constructor).Operator( fullOperator )
+    return load(includes, typeName, constructor, solve).Operator( fullOperator )