Issue with diff function, symbolic toolbox, three tiny lines of code to illustrate!

Hi there!

I'm trying to use the diff function in Matlab for a very, very complicated piece of code. There's an issue, however. Suppose I have an arbitrary function, say, a(x). I want to symbolically take lots of derivatives of a(x), and then substitute in for various x's later. This is a problem though.

For example

syms x y z
syms a(u)
diff(a(x),x)

This is wrong. What I want is the answer I get from this

syms u
syms a(u)

diff(a)

ans(u) =

D(a)(u)

Except I want to be able to do that for many, many 'u's. I know there must be a simple solution to this problem, but I can't figure it out.