:_ appears to mark it as a generic type parameter, the `hello` function in the example ought to be equivalent to this straight C++:

  template<typename T>
  void hello(T msg) {
    std::cout << "Hello " << msg << "\n";
  }

  hello: (msg: _) = {
    std::cout << "Hello " << msg << "\n";
  }

I kind of like this, actually, because so many people I've worked with are afraid of templates in C++, but they seem to have no trouble with essentially equivalent code using generics (in other statically typed languages) or dynamically typed languages where it "just works" (until it doesn't). Not that we'll be adopting this any time soon, but I suspect they'd be more amenable to this form when `template` seems to make them quake in their boots.