GOP frontrunner Donald Trump trolled Sen. Ted Cruz (R-TX), his chief rival, again on Twitter on Saturday over whether Cruz meets the U.S. Constitution’s “natural born citizen” requirements of a president.

“Constitutional law expert  #Laurence Tribe of Harvard says “wrong to say it (natural born citizen) is a settled matter-it isn’t settled),” Trump tweeted on Saturday.

Trump was citing comments that Laurence Tribe—a high-profile liberal constitutional law professor at Harvard who taught both Cruz and President Barack Obama among other high profile figures—recently told ABC News that he does not believe the natural born citizen question is “settled law.”

“I don’t agree that it’s ‘settled law,’” Tribe said. “The Supreme Court has never addressed the issue one way or the other, as I believe Ted ought to know.”

Tribe added that he personally believes Cruz is eligible, but that doesn’t mean it’s “settled law.”

“My own view as a constitutional scholar is that the better view — the one most consistent with the entire Constitution — is the broader definition, according to which Cruz would be eligible,” Tribe said, noting that he believes that a natural born citizen should include, as ABC News wrote, “anyone who is a U.S. citizen at birth and doesn’t need to be naturalized.”

Cruz was born in Calgary, Alberta, Canada, on Dec. 22, 1970. As two powerful exclusive pieces from Breitbart’s Joel Pollak show, Cruz’s mother and father were listed on an electors list in 1974 in Canada. Cruz’s father, Rafael Bienvenido Cruz, had already publicly confirmed he applied for and received Canadian citizenship back when they lived in Canada in the 1970s.

The issue of Cruz’s mother, however, had never been answered before and it had never been clear whether she had applied for or received Canadian citizenship until this week when Cruz’s campaign finally confirmed she was only in Canada on a work permit and never applied for or received Canadian citizenship.

Cruz’s campaign also provided Breitbart News with his mother’s birth certificate, proving she was born with U.S. citizenship in Delaware. All of that means that Cruz was born with U.S. citizenship since he was born to one U.S. citizen parent.

That still has not answered the question of natural born citizen, however, and even though many legal experts believe what Tribe does—that being a U.S. citizen at birth makes someone a natural born citizen—there are definitely also some who believe the definition should be more strict and based off the mid-1700s book The Law of Nations by Emer de Vattel.

“The natives, or natural-born citizens, are those born in the country, of parents who are citizens,” Vattel’s book reads.

Under that definition, since Cruz was born in Canada—and also importantly since his father was not a U.S. citizen—he would not be a natural born citizen.

That stricter definition of natural born citizen would also, interestingly, mean that Sen. Marco Rubio (R-FL) is not a natural born citizen either. Both of Rubio’s parents were naturalized as U.S. citizens in 1975—after his birth in 1971.