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import scala.language.higherKinds
class Cartesian[F[_]] {
trait Arbitrary1
trait Arbitrary2
implicit val fa1: F[Arbitrary1] = null.asInstanceOf
implicit val fa2: F[Arbitrary2] = null.asInstanceOf
class QED
implicit def proof(implicit fa12: F[(Arbitrary1, Arbitrary2)]): QED = null
trait Foo[T]
object Foo {
implicit def pair[A, B](implicit fa: Foo[A], fb: Foo[B]): Foo[(A, B)] = new Foo[(A, B)] {}
object Test {
object FooIsCartesian extends Cartesian[Foo]
def test(implicit fc: FooIsCartesian.QED): Unit = ()
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