         LaplacianM_perp_phi ( g,p.bc_x_phi,g.bcy(), dg::normed, dg::centered)
     {
     }
-    void operator()( std::vector<container>& x, std::vector<container>& y)
+    void operator()( const std::vector<container>& x, std::vector<container>& y)
     {
         /* x[0] := N_e - (bgamp+profamp)
            x[1] := N_i - (bgamp+profamp)

    const std::vector<container>& potential( ) const { return phi;}
    void initializene( const container& y, container& target);

    void operator()( std::vector<container>& y, std::vector<container>& yp);

    double mass( ) {return mass_;}
    double mass_diffusion( ) {return diff_;}

}

template<class Grid, class Matrix, class container>
void Feltor<Grid, Matrix, container>::operator()( const std::vector<container>& y, std::vector<container>& yp)
{

    dg::Timer t;
