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One of the Zermelo-Fraenkel axioms of set theory, which states that whenever \(x\) is a set and \(\mathscr{R}\) is a functional relation on \(x\), the image of \(x\) under \(\mathscr{R}\) (the set of all \(b\) such that \(a\mathscr{R}b\) for some \(a\in x\)) is a set: \(\forall x:\left[\forall a\in x:\exists!b:a\mathscr{R}b\right]\Rightarrow\left[\exists y:\forall b:b\in y\Leftrightarrow \exists a\in x:a\mathscr{R}b\right]\). This is actually an axiom schema (as it holds for every relation \(\mathscr{R}\)) and underlies the principle of restricted comprehension.