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The idea is to use some appropriate coding. A primitive recursive encoding of pairs is an encoding $\mathbb{N}^2 \to \mathbb{N}$, denoted by $\langle x,y \rangle$, such that the following functions are recursive: $e(x,y) = \langle x,y \rangle$, $p_1(\langle x,y \rangle) = x$, $p_2(\langle x,y \ra...