Reflections: Abstract there must be an upper bound? There must also be a lower bound? What criteria?
Abstract form a hierarchical structure, but not the more powerful abstract appointment, after all, always with other local phase anomalies between the bottom of the same level, after all, the more senior, the less in common;
then you need to weigh a degree , then the code looks very clear, but also to look at the structure is simple, further, extended (a bottom layer of the same level already exists) is easy;