// Look up a previously instantiated SubSequence relationship.
 public static SubSequence find(PptSlice ppt) {
   Assert.assertTrue(ppt.arity() == 2);
   for (Invariant inv : ppt.invs) {
     if (inv instanceof SubSequence) return (SubSequence) inv;
   }
   return null;
 }
 // Look up a previously instantiated invariant.
 public static PairwiseIntLessThan find(PptSlice ppt) {
   Assert.assertTrue(ppt.arity() == 2);
   for (Invariant inv : ppt.invs) {
     if (inv instanceof PairwiseIntLessThan) return (PairwiseIntLessThan) inv;
   }
   return null;
 }