I want to resolve a shortest path between two nodes (vertices) in a   
   graph; some arcs (edges) are "locked"; the locked arc can be traversed   
   only if we have already visited one of its key-nodes (and a lock can   
   have more than one key-node); the key-nodes are dispersed throughout   
   the graph.   
      
   I think this is closely similar to a SOP, but of course it is not   
   technically a SOP: we want the shortest path to the goal node, not the   
   shortest path through all the locked arcs and key-nodes.   
   I'd greatly appreciate any suggestions.   
      
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