code: public float similarity(K what, K with)
      {
          Vector vectorWhat = storage.get(what);
          if (vectorWhat == null)
          {
              return -1f;
          }
          Vector vectorWith = storage.get(with);
          if (vectorWith == null)
          {
              return -1f;
          }
          return vectorWhat.cosineForUnitVector(vectorWith);
      }
docs: 余弦相似度
  
  @param what 一个词
  @param with 另一个词
  @return 余弦相似度