function [xdot] = nsds_hw1_4m(t,x,trev)
%
%  This function models the Van-der-Pol oscillator
%  with time reversal at time trev
%
mu = 2;
%
if t < trev,
    c = +1;
else
    c = -1;
end
%
x1 = x(1);
x2 = x(2);
%
x1dot = c*x2;
x2dot = c*(mu*(1 - x1^2)*x2 - x1);
%
xdot = [ x1dot ; x2dot ];
%
return
