About: Nested set

In a naive set theory, a nested set is a set containing a chain of subsets, forming a hierarchical structure, like Russian dolls. It is used as reference-concept in all scientific hierarchy definitions, and many technical approaches, like the tree in computational data structures or nested set model of relational databases. Sometimes the concept is confused with a "set of sets" with a hereditary property (like the finiteness in a hereditarily finite set).