Run-time problem

P53 - Only backdrops can be moved to multiple places

Most problems are detected when Inform translates the source text, but this is one of the exceptions, a "run-time problem" which emerges only during play.

Most objects can only be in one room at a time. The only exceptions are two-sided doors (which are never allowed to move), and backdrops. A backdrop can be moved to occupy all rooms matching a given description, but nothing else can.