<p>The Saudi crown prince was visiting at a sensitive moment for the Middle East, so the White House plotted every detail of his weekend trip to Texas. After meeting with President Bush at his ranch, Crown Prince Abdullah was feted by the president's father at his presidential library in College Station. Then, to the dismay of White House planners, he saw an unscheduled and unexpected visitor: Bill Clinton. The former president, in Houston for a fundraiser and staying at the same posh hotel as the Saudi leader, headed upstairs after midnight for a private chat that had been arranged only a few hours earlier. Their 90-minute meeting didn't end until 2 a.m. Presidents are members of one of the world's most exclusive fraternities, those who know firsthand the challenges of the Oval Office. You might think that an incumbent would see his predecessors as valuable resources, as repositories of knowledge and experience, as potential advisers and envoys. You would be wrong.</p>