Question: Order of an ODE with respect to different functions

I have an ODE containing different orders of differentiation of f1(x), f2(x) and f3(x).

> with(PDEtools):

> difforder(ODE);

or

> difforder(ODE,x);

gives the order with respect to x but I need a "COMMAND" doing below,

> ODE:=12*diff(f1(x),x$5)^(7/2)*diff(f2(x),x$4)^9+12*diff(f3(x),x$3)*diff(f1(x),x$2)^9;

> COMMAND (ODE,f1(x));

5

> COMMAND (ODE,f2(x));

4

> COMMAND (ODE,f3(x));

3

 

 

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