code: public float similarity(String what, String with)
      {
          Vector A = query(what);
          if (A == null) return -1f;
          Vector B = query(with);
          if (B == null) return -1f;
          return A.cosineForUnitVector(B);
      }
docs: 文档相似度计算
  @param what
  @param with
  @return