nullity, its validity can be questioned at any stage. It was observed: “26. Thus it is settled law that normally a decree passed by a court of competent jurisdiction, after adjudication on merits of the rights of the parties, operates as res judicata in a subsequent suit or proceedings and binds the parties or the persons claiming right, title or interest from the parties. Its validity should be assailed only in an appeal or revision as the case may be. In subsequent proceedings its validity cannot be questioned. A decree passed by a court without jurisdiction over the subject matter or on other grounds which goes to the root of its exercise or jurisdiction, lacks inherent jurisdiction. It is a coram non judice. A decree passed by such a court is a nullity and is non est. Its invalidity can be set up whenever it is sought to be enforced or is acted upon as a foundation for a right, even at the stage of execution or in collateral proceedings. The defect of jurisdiction strikes at the