Also from Mr. Lippert’s immutability discussion (specifically the comments to post #4, “An Immutable Queue“): we have a Carnegie Mellon dissertation — I’m telling you, those CM guys are, collectively, The Man — on Purely Functional Data Structures (PDF) from one Chris Okasaki (now a professor at West Point, apparently), the book version thereof, and for those of shorter attention span some Power Point slides on the same topic from one Benjamin Pierce at the University of Pennsylvania. Go nuts, Dan!