Balls are hidden in the box. Fire lasers in from its edge, and find the balls from what each beam does. A beam goes straight across unless:
Otherwise a number marks where it went in and where it came out.
On a touch screen, a tap does what a click does, and a long press what a right-click does (more ways to right-click).
Click a square to send a beam into the box from a square around its edge, or place or remove a guessed ball on a square inside it. Right-click it to mark a square inside the box as definitely known, so it takes no ball, or a square around the edge to mark the whole row or column it looks into. Keep the button down and drag to do the same to every square you pass over that looked the same as the first.
With the keyboard, the arrow keys move a cursor around the board. Enter does what a click does to the square under it, and Space what a right-click does.
A beam’s result is ‘H’ (the beam hit a ball dead-on and stopped), ‘R’ (the beam was either reflected back the way it came or there was a ball just to one side of its entry point) or a number appearing in two squares (indicating that the beam entered one of those squares and emerged from the other). Clicking a beam you have already fired lights its result up, at both of its squares when it is a number, so you can find where it came out. Its path through the box is never drawn.
When you have placed enough balls, a green button will appear in the top left; click that to indicate that you think you have the answer. The keyboard’s cursor moves around the edge of the box as well as inside it, and onto the button once it shows.
A square marked as known takes no ball until you right-click it again.
Every board has exactly one answer: once every laser is fired, only one set of balls sends them all where they go. So Check & save can check your marks against it, as it does in every other puzzle, and highlights a ball on a square that holds none and a square marked as known that holds a ball. A game ID whose lasers allow more than one answer won’t open.
Hint reads only what your lasers have shown, never the hidden balls, so it tells you nothing you could not work out yourself.
Its main tool is following a laser through the box. Start at a laser you have fired and walk its path through the squares you already know, up to the first square you know nothing about. Ask what that square could hold: if a ball there would send the laser somewhere it did not go, the square must be empty, and if leaving it empty would, it must hold a ball. A laser that came out at a numbered square can be followed from either of its two numbers, since a path runs the same both ways.
The hint marks an empty square as known and puts a ball on a square that must hold one, so each later step can build on what is on the board. When nothing more can be settled, it asks you to fire a laser whose path still runs through squares nothing has settled.
Once every laser is fired, it can happen that no single laser settles any more squares, though all of them together do. The hint then finds balls that send every laser where it went by trying, and offers them as one run of moves. A square no laser ever reaches is settled by the count alone: when the box must hold more balls than the lasers account for, every such square holds one, and the hint asks you to put them on.
These parameters are available from the ‘Custom type…’ option on the ‘Type’ menu.
3-6 for a number picked at random from that range, which you then have to find out as you play. There must be at least one ball, and fewer balls than squares.