Collecting bounded call sites

Stream: Is there code for X?

[Leni Aniva]

Is there a function for collecting bounded call sites from a function? For example, given a expr

`2, fun x => `1 x + `4

The function should give [0, 2]. The index 3 is not included since it is not a call site.

If such a function does not exist, is there a function similar to Expr.foldlM but that is aware of the surrounding context?

[Kim Morrison]

What do the backticks represent in what you've written above? I'm not understanding!