@mlinksva @Marcel Weiher Still can't find the one I'm looking for, but #codon is another recent "slight superset to a subset" of Python approach as opposed to Mojo's "add new stuff" approach to modifying Python.

https://docs.exaloop.io/codon/general/faq

It looks to me, just at a quick glance, that Mojo syntax additions to Python could have been Python-compatible additions in the form of decorators that are noop or validating at runtime on CPython, perhaps validating on mypy, and allowing tight compilation on mojo.