% Build additional vector for momentum mx equation

Mxf=-lambda*([pold;pright]-[pleft;pold])+ ...
                  [0.5*lambda*uxleft*(1+vecsignsleft)*mxleft; ...
                   zeros(J-1,1); ...
                    -0.5*lambda*uxright*(1-vecsignsright)*mxright];


