function varargout = Xchaos(t,y,flag)
%
%    this file modelled after "help odefile"

switch flag
case ''                      % Return dy/dt = Xf(t,y).
   varargout{1} = Xforce(t,y);
case 'events'                % Return [value,isterminal,direction].
  [varargout{1:3}] = events(t,y);
otherwise
   error(['Unknown flag ''' flag '''.']);
end

% -------------------------------------------------------------

function step = Xforce(t,y)

global par

step = zeros(size(y));

step(1) = 2          + par(1)*4     *y(4)*(cos(  (y(1)-y(3))))^2;
step(2) =              par(1)*4*y(2)*y(4)* sin(2*(y(1)-y(3)))   ;
step(3) = 1 + y(4)^2 + par(1)*4*y(2)     *(cos(  (y(1)-y(3))))^2;
step(4) =            - par(1)*4*y(2)*y(4)* sin(2*(y(1)-y(3)));

step(1) = step(1) + par(2)     *sin(y(1));
step(2) = step(2) - par(2)*y(2)*cos(y(1));

% -------------------------------------------------------------

function [value,isterminal,direction] = events(t,y)

global skip

value = cos(y(1)/skip);   isterminal = 1;   direction = -1;