ztrsen(3) on Linux
The text c0b34f8abeeb, as Debian 3.0 (Woody) shipped it.
reorder the Schur factorization of a complex matrix A = Q*T*Q**H, so that a selected cluster of eigenvalues appears in the leading positions on the diagonal of the upper triangular matrix T, and the leading columns of Q form an orthonormal basis of the corresponding right invariant subspace