General

• Rewrite exact_cover_dlx.py in C as an extension module.
• Complete unit tests.
• Implement a graphical front-end. Ideas:
  • The ability to turn pieces on or off, and control the number of instances of each piece.
• Restore state from a solution file instead of from a saved state file? I.e. use the last solution in the solution file to simulate a saved state file.
• Add multiprocessing support.

Puzzles

See the "Potential Puzzles" sections of the Notes for each polyform family:

• Notes on Polyominoes
• Notes on Polyiamonds
• Notes on Polyhexes
• Notes on Polycubes
• Notes on Polysticks
• Notes on Polytrigs
• Notes on Polytwigs