Author: Anurag Sahay
Blacken some cells in the grid so that each arrow (including blackened) shows to exactly one blackened cell.
Grid control: Click the mouse button at the selected cell to change the color of the cell. Second click will paint the cell into auxiliary grey color, which doesn't affect correctness of the solution, but helps visually. You can also drag the content of one cell to another with the mouse. More details about grid control are at the help page.