Rules:
1. The grid is divided into rooms (thick-border regions).
2. Place exactly ONE diagonal mirror (/ or \) in each room.
3. Around the outside are letter pairs (A-A, B-B...). Each letter has a number = beam reflection count.
4. A beam enters at one letter (perpendicular to edge), bounces off mirrors, exits at the matching letter.
5. Every mirror must be hit by at least one beam.
Controls:
- Click a room to place the selected mirror.
- Left-click for \, Right-click for /.
- Keys: 1=place \, 2=place /, 3=erase, H=hint, R=reset, Enter=check, Esc=menu.