docs: function (puzzle) {
                  for (var i = 0, height = puzzle.length; i < height; i++) {
                      var row = puzzle[i];
                      for (var j = 0, width = row.length; j < width; j++) {
                          if (!puzzle[i][j]) {
                              var randomLetter = Math.floor(Math.random() * letters.length);
                              puzzle[i][j] = letters[randomLetter];
                          }
                      }
                  }
              }