A functional predicate is a predicate in two variables (in this case, and ) such that and together imply . If holds, then we may write .
Note that this permits us to speak of "functions" which act on all sets. In set theory, the relation , for example, cannot yield a function unless it is confined to a specific domain and range. However, we may speak of the functional predicate , which may be applied to any set. This permits us to speak of general functions on sets within the context of classical set theory.
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