  We prove an asymptotic for the number of additive triples of bijections
$\{1,\dots,n\}\to\mathbb{Z}/n\mathbb{Z}$, that is, the number of pairs of
bijections $\pi_1,\pi_2\colon \{1,\dots,n\}\to\mathbb{Z}/n\mathbb{Z}$ such that
the pointwise sum $\pi_1+\pi_2$ is also a bijection. This problem is equivalent
to counting the number of orthomorphisms or complete mappings of
$\mathbb{Z}/n\mathbb{Z}$, to counting the number of arrangements of $n$
mutually nonattacking semiqueens on an $n\times n$ toroidal chessboard, and to
counting the number of transversals in a cyclic Latin square. The method of
proof is a version of the Hardy--Littlewood circle method from analytic number
theory, adapted to the group $(\mathbb{Z}/n\mathbb{Z})^n$.
