[math-fun] nice little sliding block head exercise

Neil is tabulating the "hardest" 4x4s according to polyomino size histogram. Some of the short ones are tricky. E.g., a four-piecer: http://gosper.org/4x4-3,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0.gif . Five moves to get the tetromino to the lower right, where a move is of one piece along any path. --rwg