Non wellfounded mereology

In philosophy, specifically metaphysics, mereology is the study of parthood relationships. In mathematics and formal logic, wellfoundedness prohibits ⋯ < x < ⋯ < x < ⋯ for any x.

Thus non-wellfounded mereology treats topologically circular, cyclical, repetitive, or other eventual self-containment.

More formally, non-wellfounded partial orders may exhibit ⋯ < x < ⋯ < x < ⋯ for some x whereas well-founded orders prohibit that.