I. Introduction
Sudoku is currently one of the most famous puzzles in the world. The most popular version consists on a grid made up of subgrids, but the general case, an grid with subgrids is considered. Some cells contain numbers, which can be considered as input data. The goal is to fill in the empty cells, one number in each, so that each column, row, and subgrid contains the numbers 1 through 9 exactly once (numbers 1 to in the general case). If the input data are correct, the sudoku has one and only one solution.