
function [c, f, s] = demo4fun(x,t,u,DuDx)

c=[1,1] ; 
f = [0,0] ;

% s(1) = DuDx(2) ;    % move right without change of form 
% s(2) = DuDx(1) ; 

% s(1) = DuDx(2) - 3*u(2) ;   % packet and waves move rightwards
% s(2) = DuDx(1) + 3*u(1) ; 

% s(1) = DuDx(2) + 3*u(2) ;   % packet and waves move leftwards relative to packet
% s(2) = DuDx(1) - 3*u(1) ; 

 
