ctgsen(3) on Linux
The text 82c2d1ede89f, as Debian 3.1 (Sarge) shipped it.
reorder the generalized Schur decomposition of a complex matrix pair (A, B) (in terms of an unitary equivalence trans- formation Q' * (A, B) * Z), so that a selected cluster of eigenvalues appears in the leading diagonal blocks of the pair (A,B)