[still_grokking]

That's obviously the wrong signature.

The semantically correct one is "Container[T : Printable].join(): String"; T must implement the Printable concept, but Python lacks concepts (or type-classes as these are alternatively called).

But this all is irrelevant as this is anyway not the signature of `str.join()` in Python. It's `str.join(Iterable[str]) -> str` there. With sane design and syntax it would just become `Iterable[str].join(str)`.