What's the function of a function variable's function?

[Han]

You can find more info by looking at the documentation for xcas/giac.

There isn't anything special about a variable whose content is a function. It's supposed to reflect a mathematical function. So you can create functions by simply using the form:

function_name(var_name):= <expression in terms of var_name>;

This is typical for any CAS. The CAS also knows how to handle functional algebra. So you can define two functions f(x) and g(t) and ask the CAS to compute h:=f+g, and then use compute, say, h(3).