Ah. I guess that. Thank you very much for detailed information!
It's really unfortunate because multi-argument functions don't work really. It was a mistake. There is solid algebra for type system design, and it should be followed. Most of the "features" people want can be made to work as sugar on top of this for direct calls. The sugar doesn't work when you move to closures which is just fine.
Anyhow as John McCall says, and i said myself, its unlikely the mistake can be fixed now, which unfortunately dead ends any further higher level extensions.
However it IS pertinent. If the labels on tuples are going away, the equality and total ordering and conversion to string and any other function with an invariant return type can be implemented by the user provided protocols support polymorphic recursion and tuples have a head/tail representation. This means it doesn't have to be special cased in the compiler, and can be defined in Swift instead, in the standard library or in user code.