MuPAD: Differentiation w.r.t. functions.

Hello,

I would like to "linearize" a nonlinear differential equation arouns some
"operating point". For example:

>> deq := diff(x(t),t)^2+x(t)*cos(y(t))=u(t)

To do that, I need to find the first terms of multivariate Taylor expansion,
that is, I need to find diff with respect to "variables" dot(x)(t), x(t),
y(t).

How can I do that? Of course, a working way is to substitute "expressions"
for "functions" like:

>> subs(deq,diff(x(t),t)=x_dot)

and then to use diff or even linalg::jacobian w.r.t. variables x, x_dot, y

But is there a more elegant way? The thing is that I have many variables in
the equation and I don't feel like doing the above described substitution
by hand.

Thanks,
Zdenek Hurak
.