Judgment has been reserved in an appeal by the North's First Minister, Mr David Trimble, against a High Court ruling that he acted unlawfully by banning Sinn Fein ministers Mr Martin McGuinness and Ms Bairbre de Brun from attending NorthSouth Ministerial Council meetings.
The ban was implemented because Mr Trimble believed Sinn Fein was not putting enough pressure on the IRA to decommission its weapons.
After a day of submissions on the wording and meaning of both the Belfast Agreement and Northern Ireland Act, the Lord Chief Justice, Lord Carswell, said a judgment would be delivered as soon as possible.
Earlier, Mr Michael Lavery, counsel for Mr McGuinness, argued that Mr Trimble could not "have the best of both worlds".
"We can't have a situation where the First Minister says `I'm going to take it upon myself to police this agreement. I'm going to take it upon myself to appoint - or not appoint - ministers', " he added.
That claim was rebuffed by Mr Trimble's counsel, Mr Declan Morgan: "He [the First Minister] doesn't police the agreement because he doesn't say a minister is a minister. But he can work within the scope of his discretion."
The interpretation of that discretion by High Court judge Mr Justice Kerr in the original hearing is at the core of both the First and Deputy First Minister's appeal cases.
The Deputy First Minister, Mr Seamus Mallon's counsel, Mr Peter Smith, told the court that Mr Justice Kerr had misunderstood the difference between an Assembly minister's vital role as a participating decision-maker on cross-Border bodies, as opposed to fulfilling only a representative function.