Step 2 of 8
"0" means mark them all
The friendliest pillar on the board: it decides its neighbors on sight.
The rule of thumb: A "0" pillar wants no torches beside it — mark every neighbor X on sight and move on.
A "0" pillar wants no torches beside it, so every white neighbor gets an X the moment you spot it. No counting, no thinking: mark and move on.
Those X’s earn their keep. Each one shrinks the candidate list of a nearby dark room and the open neighbors of a nearby pillar, which is how the other rules get their grip. These practice boards are picked so the marking matters — skip the zeroes and the rest of the reasoning stalls with rooms still dark.
Where to look: Every "0" on the board, the moment you see it. Then re-scan the rooms and pillars next to the ones you just marked.
Worked example
A real position from a board that needs "0" means mark them all. Purple: what the reasoning rests on. Dashed: the cells in question. Green: what it proves.
Zeroes handled. Next: the opposite extreme, a pillar that wants everything. Step 3: "4" means light all four →