Exclusion

Exclusions are areas of the Dungeon which have been marked as especially dangerous or unwise to explore. The game will ask you for confirmation before allowing you to walk into an exclusion manually.

Autoexplore will mark exclusions whenever there is stationary danger, such as an oklob plant or dangerous cloud areas. You can create exclusions by entering examine mode (press x), moving the cursor over the tile you wish to exclude, and then pressing e. Pressing e once will exclude all terrain within line of sight of the excluded tile. Pressing it again will only exclude the tile itself, and pressing it a third time will erase the exclusion altogether. Pressing Ctrl-E will remove all exclusions on the current floor.

Strategy

• Placing a line of sight exclusion on a sleeping monster you hope to avoid (hydras, dangerous uniques, etc.) will allow you to continue exploring a floor with auto-explore without the risk of waking it up. Bear in mind that there are many ways to generate noise that may wake it up, even if it's not currently visible.
• If you've discovered a vault that you don't want to deal with immediately, placing a single tile exclusion on the door to it will ensure that you don't accidentally open it prematurely.
• Players who tend to Tab their way through combat may wish to use single tile exclusions when involved in difficult battles near shallow water, webs, or other terrain that one may accidentally step into when automating combat.
• Excluding all but one of the staircases on a floor will guarantee that you always use the remaining one when auto-travelling. This is useful when trying to minimize the time it takes to reach your stash.
• Excluding stairs will give you a warning when you attempt to go up/down that staircase. This is useful to prevent yourself accidentally going downstairs to a dangerous pack of monsters or other danger.