[wool_gather]

There's certainly no reason to use reference equality. In fact, I think equality is the wrong question: it should duplicate the match. `case (x, y, x)` should `__match__((a, b, c))` IFF `a.__match__(c)` (Not sure if that's exactly the right syntax for the Python calls, but hopefully the idea is clear.)

(Elixir has a similar operation to "pin" in a match: if you use an existing variable as the pattern, but pin it, the value must match whatever the variable is already bound to.)

> I think at least making it an error now can leave it open to defining it later.

Fair point.