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Goal

Fill in grid cells so digits 1 through 9 occur once in each row, column, and 3x3 box.

Game menu

New game - start new game
Restart game - replay current game
Solve game - solve current game

Enter digits

Select cell, then tap desired digit button
or
Select digit button, then tap one or more desired cells

Delete digits

Select cell, then tap ? or tap same digit button as cell
or
Select ?, then tap one or more cells

Undo and Redo

Tap Undo to undo last digit entry or deletion
Tap Redo to restore the last undo

Enter pencil marks

Select ?, then select cell, then tap digits
or
Select ?, then select digit, then tap cells

Delete pencil marks

Select ?, then select cell, then tap digits to be removed (tap ? to remove all)
or
Select ?, then select digit, then tap cells to remove that digit
or
Double-tap ? to remove marks in all cells

Automatic pencil marks

With Auto ? off, pencil marks are entered manually. Select Auto ? to mark in all cells automatically. Auto marks update as digits are entered or erased and can be further deleted manually. Unselect Auto ? to turn off auto mode and return to manually entered marks. When Auto ? is on, double-tap ? to restore erased marks in all cells

Settings menu

Help - display help (this screen)
Reset stats - zero out all statistics
Digit highlighting - highlight all cells with currently selected digit as entry and/or mark
Error indicators - indicate which cells are in conflict and/or incorrect
Color schemes - select desired color motif
Hint display - select whether hints display for only next entry or are continuously updated

Digit colors

5 (black) = clues
5 (green) = correct entries
         (if Error indicators = Errors or Both)
5 (red) = incorrect entries
         (if Error indicators = Errors or Both)
5 (blue) = all entries
         (if Error indicators = Conflicts or Off)

Mark colors

5 (purple) = marks entered with auto ? off
5 (blue) = marks determined by auto ? on
5 (pink) = marks deleted with auto ? on

Cell background colors

      = cell selected for entry of digit or marks
      = cell contains currently selected digit

    (if Digit highlighting = Digits or Digits & Marks)
      = cell marks include selected digit

    (if Digit highlighting = Digits & Marks)
      = duplicate digit in row, col, or 3x3 box

    (if Error indicators = Conflicts or Both)

Digit button colors

= digit button not selected
= digit button selected
= digit completed

Hints menu

Correct so far? - report whether or not the grid has errors
Suggest technique - suggest technique for finding next easiest to solve
Suggest digit - highlight digit that is next easiest to solve
Suggest cell - highlight cell that is next easiest to solve
Reveal one answer - fill in the answer for some cell
No hint - turn off hint display

Bookmarks menu

Set bookmark - remember game at this time
Back to bookmark - revert game to the last time Set bookmark was pressed
Back to last correct - revert game to last time all entries were correct

Tip: Using digit highlighting

For no visual assistance in seeing digits, turn Digit highlighting to Off.

For more assistance, set Digit highlighting to Digits, tap a digit, and use highlighted values to identify other cell locations for this digit.

For the most assistance, set Digit highlighting to Digits & Marks and turn on Auto ?. Now tapping a digit will help identify rows, columns, or 3x3 boxes with single remaining locations for this digit (hidden singles).

Tip: Using error indicators

For no visual assistance in seeing errors, turn Error indicators to Off.

Setting Error indicators to Conflicts (or Both) will reveal when the same digit entered twice in the same row, column, or 3x3 box.

Setting Error indicators to Errors (or Both) will reveal whether an entry is correct (green) or incorrect (red).

Tip: Using hints

The commands Correct so far?, Suggest technique, Suggest digit, Suggest cell, and Reveal one answer provide an increasing amount of information that can be used to make progress.

The Hints commands first look for the last empty cell in a row, column, or 3x3 box, then a single candidate remaining in a row, column, or 3x3 box (hidden single), then a single candidate remaining in a cell (naked single), then cells with the least remaining candidates.

Tip: Using pencil marks

Pencil marks can be entered manually, automatically, or in combination.

With Auto ? off, pencil marks can be entered manually to indicate possible values in a cell or possible cell locations for a value.

Turning Auto ? on saves any current manual marks and marks in all candidates in all cells. As digits are entered or erased, marks in related cells are removed or added back, so that auto pencil marks are always correct.

Pencil marks can be manually erased (made dim) while Auto ? is on. A technique often used in harder games is to rule out candidates one at a time in cells which can't be solved.

When Auto ? is turned back off, the previously saved manual marks are restored.

Tip: Using bookmarks

For more difficult puzzles, the trial-and-error or "Nishio" method is popular. When using this technique, Error indicators should not be set to Errors or Both, otherwise correct and incorrect entries will be revealed immediately. Also Auto ? is usually turned on as an aid.

Upon reaching a point in the game where guessing is desired, find a cell with, say, two candidates. Tap the cell to select it, then tap Back and then Set bookmark to remember the current state of the grid and this selected cell.

Now enter one of the candidate values into this cell and continue playing the game, either to a successful conclusion or to a dead-end. If at a dead-end, tap Back and then Back to bookmark to revert the grid back to the first bookmark with the previously selected cell highlighted. The second candidate can now be confidently entered into this cell, since the first guess led to a dead-end.

Multiple bookmarks are handled as a stack so that all possibilities at multiple bookmarks can be explored by backtracking. Each Back to bookmark command reports the number of previous visits to this bookmark to help with which guess to try next. If after going Back to bookmark there are no more candidates to try at that bookmark, tap Back to bookmark a second time to go back to earlier bookmarks.

A shortcut to backtracking is to select Back to last correct, which reverts the game to the last time all entries were in fact correct.

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