What is the ruling on the imposition delivered by a concealed witness against a defendant, upon request of a plaintiff ,where the witness can see and hear the defendant while the latter can`t and is unaware of his presence. This is because, before people, the defendant denies the plaintiff`s right, but, in private, he admits it. Therefore, the plaintiff conceals the witness to testify against the defendant, since there is no other way to prove his right?