Error "()-indexing must appear last in an index expression" in symbolic Matlab Toolbox

Hello everyone, I want to operate with some symbolic expressions and their derivatives, but I'm finding some problems when trying to do something like this:

>>syms x(t)
>>q=diff(x,t)+2
        q(t) =

        D(x)(t) + 2

>>dq=diff(q,D(x)(t))
        Error: ()-indexing must appear last in an index expression.

I want to differentiate q(t) with respect to the derivative of x(t), and I was wondering if there is any way to do this without substituting D(x)(t) for another variable

[KSSV]

what you want to do actually?

[paco ocap]

I want to solve Lagrange equations for any given lagrangian. And as I have many variables, and I dont know which of them is differentiated or not at first, I think it would be easier to solve things following the process showcased in the question. But if it is not possible, I will try to do it another way.

Answer 1

[Mischa Kim]

Paco, check out the Euler-Lagrange tool on File Exchange. It does exactly what you are trying to do.