### symbolic differentiation of unknown function

I want to do some formal calculus with ~~a ~~unknown ~~functions ~~functions
for the purpose of solving differential ~~equations~~equations.

Say

`F(t) = `~~v(t)*t^2, ~~v(t)*t^2

, where ~~v ~~`v`

is an unknown differentiable ~~function. ~~function.

Then I would like to declare ~~v ~~`v`

as such and be able to get

`F.diff(t) = `~~2~~*t*v+t^2*v.diff(t)2*t*v+t^2*v.diff(t)

It is similar to ~~this question, ~~Ask Sage question 8822 but the solution does not seem to work anymore, as ~~function() takes ~~`function()`

takes
only one argument and not 2 as in the ~~description;
ask.sagemath.org/question/8822/is-there-any-way-to-define-an-as-yet-unknown-function/~~description.

Anyone know what the syntax is in 8.9? Or in 9.0, when that becomes available?