   const RealType& hxSquareInverse = entity.getMesh().template getSpaceStepsProducts< -2, 0 >(); 
   const RealType& hySquareInverse = entity.getMesh().template getSpaceStepsProducts< 0, -2 >(); 
   const IndexType& center = entity.getIndex(); 
   const IndexType& east  = neighbourEntities.template getEntityIndex<  1,  0 >(); 
   const IndexType& west  = neighbourEntities.template getEntityIndex< -1,  0 >(); 
   const IndexType& north = neighbourEntities.template getEntityIndex<  0,  1 >(); 
   const IndexType& south = neighbourEntities.template getEntityIndex<  0, -1 >();         
   return ( u[ west ] - 2.0 * u[ center ] + u[ east ]  ) * hxSquareInverse +
          ( u[ south ] - 2.0 * u[ center ] + u[ north ] ) * hySquareInverse;