Everything today is faster than it used to be, but this seems astonishing: Historians already are shaping their assessments of Barack Obama's presidency.Obama apparently tempted fate when he made an astute comment to David Remnick, editor of The New Yorker: "At the end of the day, we're part of a long-running story. We just try to get our paragraph right."I've often written that history changes with time, and so I think the president requires three paragraphs.• Here's what might have been written as the president took office:At the end of the first decade of the 21st century, the United States elected its first black president. Barack Obama of Illinois wasn't so much a politician as a phenomenon.