Thanks Tom. I'll work up the code, and see if I can figure out some   
   heuristics and preprocessing given the additional locked-door   
   constraint.   
      
   > May I ask what this is for?  Sounds like a game level designer's   
   > helper, but maybe that's just me.   
      
   It is a part of an AI Agent for an interactive fiction system.   
   "Inter-room navigation/key acquisition" is just a basic part of the   
   system, which also involves other constraints which can be sequentially   
   ordered (such as acquiring an object, attacking, etc.), which is one   
   reason I'm looking into SOP (and 0-1 MILP). The system needs to be   
   pretty fast, since we want to support numerous Agents simultaneously,   
   without any real noticible calculation time lag.   
      
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