Doors

A door is its own entity, referenced from an exit line with through: the exit declares the geometry once and the reverse direction is inferred. The door block itself never names its rooms: a door's location is its room pair.

create the Hall
  a room
  down to the Cellar through the cellar door

  A cold entrance hall.

create the cellar door
  a door, lockable with the tarnished key
  aka grey door

  A grey door, locked as long as anyone can remember.

Defaults follow IF convention: a door starts closed (starts open overrides); lockable on a door starts locked, with the key named right on the adjective; a permanently open passage needs no door entity at all. With that one declaration OPEN, CLOSE, LOCK, and UNLOCK work and the door is visible from both sides. The full rules, including mirrored far-room lines and the diagnostics, are documented in the grammar reference.