ti-nspire defining variables

Question

I want to define a program/function which defines a function itself and the 1., 2. and 3. derivative of it as global variables. The name of the function should be variable.

Smt like:

Define LibPub der(a,b)=
Prgm
a(x):=b
a1(x):=d/dx (a(x))
a2(x):=d/dx (a1(x))
a3(x):=d/dx (a2(x))

with a being the name of the function and b being the function.

That doesn’t work because variable a cant be the name of the function. Executing this just safes a1, a2, a3