Can you clarify what you mean by containment here?
The type substitution and matching rules written down in my pitch:
If Elements = {}, then (Elements...) = (), and () doesn't match (First, Rest...) because (First, Rest...) has one non-variadic element, so it can only match a tuple of length greater than or equal to one.
(_: Int) would match (First, Rest...) with First := Int and Rest := {}, and (Int, String) would match (First, Rest...) with First := Int and Rest := {String}, for example.