//> using options -language:experimental.captureChecking
scala> import caps.*
scala> trait IO extends SharedCapability, Classifier
// defined trait IO
scala> def f(x: Object^): Object^{x.except[Control]} = ???
def f(x: Object^): Object^{x.except[Control]}
scala> def g(x: Object^): Object^{x.only[IO]} = ???
def g(x: Object^): Object^{x.only[IO]}
scala> def widen(x: Object^): Object^{x} = f(x)
def widen(x: Object^): Object^{x}
