# periodically forced predator-prey
x'=x*(a-x-y)
y'=y*(x-1/2)
a=a0*(1+f*u)
u'=u*(1-u^2-v^2)-w*v
v'=v*(1-u^2-v^2)+w*u
init u=1,x=.1,y=.1
par f=0,a0=1,w=1
done