deff('y=f(t,x)','y(1)=-x(1)*x(2)/2+x(1)/2;y(2)=x(1)*x(2)-x(2)');
T=250;N=1000;dt=T/N;t=0:dt:T;
y=ode([1.1;1.1],0,t,f);
xset('window',0);
clf;
plot2d([t',t'],[y(1,:)',y(2,:)'],leg='proies@predateur')
xset('window',1);
clf
plot2d(y(1,:),y(2,:))
