It's slightly older than Limbo, and comes from Newsqueak. The difference is that Newsqueak used keywords for the other non-variable declarations. `x: int = 123`

It's slightly older than Limbo, and comes from Newsqueak. The difference is that Newsqueak used keywords for the other non-variable declarations. `x: int = 123`, `const X: int = 123`, `type My_Int: int`, etc.