Well, maybe that's not so clear really, the logic that I meant about error
message is that it has a union type for the 3rd argument in "got" section and
not a concrete type, which obviously means compiler could not detect one
concrete binding for this argument; yet in the type for the argument there are
listed signatures for both your module's `isDigit` and `strutils`' `isDigit`,
and `strutils` is imported. With qualified name (or having renamed your proc)
you get rid of ambiguity and make the arg's type concrete; otherwise, by
turning the parameter into `untyped`, you allow unbound identifiers.
