Question: Problem with partial derivaitve of equation with dot notation



I have problem with usage of diff function. I need to compute partial derivative over  diff(varphi__l(t),t) in that equation.

T__ln := (1/2)*(diff(varphi__l(t), t))^2+((1/2*(m))*v__l+(1/2*(I))*(diff(varphi__l(t), t))^2)+((1/2*(m))*v+(1/2*(I))*(diff(varphi__l(t), t))^2)

If I am trying to use:

diff(T__ln, diff(varphi__l(t), t))

I get the error:

Error, invalid input: diff received diff(varphi__l(t), t), which is not valid for its 2nd argument

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