A final, minor addition was that of a predefined Limbo location with all the usual compass directions defined on it. That serves the dual purpose of having an "out of play" location that can be tested (and where objects can be sent if needed) and letting Alan know that 'ne' is a direction even if no exit in the game points that way. Otherwise, since exit names are entirely arbitrary in the language, it wouldn't know to tell me "you can't go that way".

the limbo isa location
  exit north, n, south, s, east, e, west, w to limbo.
  exit northeast, ne, southeast, se to limbo.
  exit southwest, sw, northwest, nw to limbo.
  exit up, u, down, d, 'in', out to limbo.
end the limbo.