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This seems to be what Common Lisp does, or am I missing some subtlety?

It seems they're trying to formalize what Common Lisp and other languages do.

It was also a suggested addition to ECMAScript 4, although I think it's out now.

Similar, but different:

I notice that they left Objective-C out of their examples, which is curious because of how close it is to what they're advocating.

What is Gradual Typing?