I have a variety of SymPy expressions involving UndefinedFunction instances:

```
f = Function('f')
g = Function('g')
e = f(x) / g(x)

```

How can I obtain a list of the function invocations appearing in such expressions? In this example, I'd like to get `[f(x), g(x)]`.

I'm aware of `free_symbols` but it kicks back `set([x])` (as it should).