Clarify construction rules for calling function in $filter

The protocol spec calls out that you can invoke a function in $filter using the namespace- or alias- qualified name, but does not clearly spell out the rules for invoking a bound function from the current resource.

Specifically, to call bound function in filter do you need to prefix with $it?
i.e., ~Products?$filter=$it/Model.GetPrice(3) gt 10
or can I do
i.e., ~Products?$filter=Model.GetPrice(3) gt 10 ?

If the latter, then how do I differentiate between a bound function and an unbound function with the same non-binding parameters?