Negative membership
Negative membership generalizes the work on multisets [for example in the author's paper, ibid. 30, 36-66 (1989; Zbl 0668.03027)], to include negative values for the multiplicity of a member. For example, this allows an absolute complement of a multiset to be defined. A model for the theory is given in ZFC.
scientific article; zbMATH DE number 4208069