Subpoena’s can be challenged. Especially one that forces someone to talk. If a challenge occurs, then Mueller has to show his hand and prove to the court, what crime was committed, what he wants and why interviewing the President of the United States gets him that information. Basically, he’ll be forced to show his hand. If he’s holding a 2,5,6,8,9, in four different suits, he’s screwed. And, this thing goes away. And he looks like a bigger corrupt, idiot than he already does.