Question: How to get a paseplan of a odesys?

here i have an ode system:

diff(x(t),t)=f(x,y,z,w); diff(y(t),t)=g(x,y,z,w);diff(z(t),t)=h(x,y,z,w); diff(w(t),t)=k(x,y,z,w) where f, g, h, k are given nonlinear functions.

Then how can i get the phaseplan of (x(t),y(t))? when i use the procedure of DEplot, it returns an error stating that the DESYS requires two or fewer variables? how can i deal with that?

Any help will be greatly appreciated!

 

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