   const RealType& lambdaX = tau * entity.getMesh().template getSpaceStepsProducts< -2, 0 >(); 
   const RealType& lambdaY = tau * entity.getMesh().template getSpaceStepsProducts< 0, -2 >(); 
   const IndexType& center = entity.getIndex(); 
   const IndexType& east  = neighborEntities.template getEntityIndex<  1,  0 >(); 
   const IndexType& west  = neighborEntities.template getEntityIndex< -1,  0 >(); 
   const IndexType& north = neighborEntities.template getEntityIndex<  0,  1 >(); 
   const IndexType& south = neighborEntities.template getEntityIndex<  0, -1 >();         
   matrixRow.setElement( 0, south,  -lambdaY );
   matrixRow.setElement( 1, west,   -lambdaX );
   matrixRow.setElement( 2, center, 2.0 * ( lambdaX + lambdaY ) );
   matrixRow.setElement( 3, east,   -lambdaX );
   matrixRow.setElement( 4, north,  -lambdaY );