Farewell to David Shribman. Before David Shribman came to the Globe, the paper had no such thing as a nonpartisan political essayist. In his nearly 10 years here, Shribman has perfected a certain type of column: gracefully written, insightful, and never mean-spirited or ideologically driven. Today's is typical. He takes an obvious story -- the pack of Democrats getting ready to challenge President Bush -- and does the atypical, making smart comparisons between now and 1992, when another pack of Democrats was getting ready to challenge an earlier President Bush, also at a time of war and economic uncertainty.