Hey Brian, I really enjoyed reading your work on the Pips game! I found myself applying a similar backtracking algorithm to my Pythonic solution (https://github.com/ematth/pips). I focused on finding a single solution for each puzzle as opposed to all possible solutions. For hard puzzles with longer run times, I found that running multiple processes, each with the domino list shuffled, gets the solve time down to <15 seconds.