Suppose that we design a building which must satisfy a lot of constraints, such as two floors, three bedrooms, the area of dining room must be greater than 10 square meters, etc. First, we have a building sketch which only satisfies some main constraints. Then, the sketch is refined. Finally, we have a complete design. From the hierarchical point of view, the building sketch can be regarded as an equivalence class consisting of all sorts of
buildings that satisfy the main constraints. Then the equivalence class is gradually partitioned into the final design via the refining of the sketch.