From: luque@info.ucl.ac.be   
      
   Dear all,   
      
   I wonder whether somebody has already solved this problem: find the   
   smallest (directed) graph, with respect to the number of edges, that   
   respects a set of constraints of the form:   
      
   - reach(x y): node y should be reachable from node x.   
   - noreach(x y): node y should not be reachable from node x.   
      
   Any relevant reference will be very appreciated!   
      
   Thanks in advance,   
      
   Luis   
      
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