0 a set (= a group of numbers) that contains no elements, represented by the symbol {} -- 空集，空集合（用符号{}表示）
Should + be used to distribute a function over an empty set of solutions, the result, too, should be empty.
If the assignment would cause a conflict with an already-assigned variable, the cache entry contains a conflict set; otherwise, it contains the empty set.
The idea is to iteratively compute, starting with the empty set, the intersection of the extensions which are compatible with the information obtained so far.
Computation of this set begins with the empty set; predicate statements are added to the set as proofs for their inclusion are found.
We emphasise that an empty slice is a valid slice, having no conclusions, and the empty set of slices is a valid net.
It also lacks a lower bound, because the empty set is not a decomposition.
Each sensible graph model interprets all the closed unsolvable terms by the empty set.
The empty set is not large because an orbit is by definition not empty.
