https://www.etsy.com/uk/listing/207657139/new-string-art-programI'm currently giving away a Sudoku program to everyone who buys a copy of my String Art computer program. It has 10,000 puzzles carefully graded in order of difficulty with five skill levels from easy peasy to rock hard. It has too many features to list here but it is very comprehensive and easy to use with many hours of puzzling guaranteed!