code: public Word<I> findSeparatingWord(final S s1, final S s2) {
  
          final FastMealyState<O> n1 = this.nodeToObservationMap.get(s1);
          final FastMealyState<O> n2 = this.nodeToObservationMap.get(s2);
  
          return NearLinearEquivalenceTest.findSeparatingWord(this.observationTree, n1, n2, this.alphabet, true);
      }
docs: Find a separating word for two hypothesis states.
  
  @param s1
  first state
  @param s2
  second state
  
  @return A {@link Word} separating the two words. {@code null} if no such word is found.