dev

1. check the correctness of the dynamic dimensionality solution
2. proofreading and adding illustrations
3. how to treat the ancestor-descendant queries under the environment with dynamic dimensionality, a compromise is to assume that the user knows the depth of the hierarchical she wants to, otherwise it becomes a full-text search
4. Khuzaima and Lei don't like the word "index", I should be careful and use another word