Pattern
<p>
Fill in the grid with a pattern of black and white squares, so that
the numbers in each row and column match the lengths of consecutive
runs of black squares.
<p>
Left-click in a square to mark it black; right-click (or hold Ctrl
while left-clicking) to mark it white. Click and drag along a row or
column to mark multiple squares black or white at once. Middle-click
(or hold Shift while left-clicking) to return a square to grey
(meaning undecided): dragging like that can erase a whole rectangle,
not just a row or column.