ABCD

You’re given an empty grid, and several rows of numbers on the side. You have to write a letter in every empty cell, following these rules:

The numbers on the edge indicate how many instances of a specific letter appear in that row or column.
Identical letters can not be horizontally or vertically adjacent.

The inventor of this type is unknown. This puzzle is also known under the names ABCD Puzzle, ABC-Kombi or ABCD-Rätsel.

Controls

ABCD uses the same control scheme as Solo, but with letters instead of numbers.

Left-click to select a cell, then type a letter on your keyboard to enter it. Press Backspace or Space to clear a cell.

Right-click a cell, then type a letter to add a pencil mark. Pencil marks can be used for any purpose.

You can also use the arrow keys to move the selected cell around. Press Enter to toggle between entering letters and entering pencil marks.

Press the ‘M’ key to fill every empty cell with all possible pencil marks.

These parameters are available from the ‘Custom…’ option on the ‘Type’ menu.

Width, Height: Size of the grid in squares (excluding the size of the numbers on the edge).
Letters: The amount of different letters that can appear in the puzzle.
Remove clues: When enabled, the difficulty is increased by hiding certain number clues.
Allow diagonal touching: When disabled, letters cannot be diagonally adjacent (in addition to letters not being orthogonally adjacent).

This puzzle is fully implemented and playable.