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In set theory, the terms "contains" and "belongs" have well-established, non-overlapping standard meanings. The predicate "contains" () is strictly a relation between two subsets of the same set: "A contains B" means "B is a subset of A". The relationship between a set and its elements () is read either "belongs" or "is in": "x belongs to A" is the same as "x is an element of A". The symmetric predicate () is rarely used, and may be read "has" or "owns", never "contains". So, to avoid conflict with the mathematical tradition, methinks that the best choice for the abstract set type operations would be