A binary relation between two sets is the subset relation, also called "set inclusion". If all the members of set formula_2 are also members of set formula_3, then formula_2 is a "subset" of formula_3, marking formula_14. For example, formula_15 is a subset of formula_1, but formula_17 is not. From this example, it is clear that a set is a subset of itself. In cases where one wishes not to have this, the term "proper subset" is meant not to have this possibility.
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