code: protected Double computeAssociation(SemanticSpace sspace, 
                                          String word1, String word2) {
          Vector v1 = sspace.getVector(word1);
          Vector v2 = sspace.getVector(word2);
          if (v1 == null || v2 == null)
              return null;
          
          // Find the ranks of each of the two words to each other
          double rank1 = findRank(sspace, word1, word2);
          double rank2 = findRank(sspace, word2, word1);
          return 2d / (rank1 + rank2);
      }
docs: Returns the association of the two words on a scale of 0 to 1.
  
  @return the assocation or {@code null} if either {@code word1} or {@code
  word2} are not in the semantic space