Non-wellfounded mereology

In philosophy, specifically metaphysics, mereology is the study of parthood relationships. In mathematics and formal logic, wellfoundedness prohibits $$\cdots<x<\cdots<x<\cdots$$ 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 $$\cdots<x<\cdots<x<\cdots$$ for some x whereas well-founded orders prohibit that.