ztgexc(3) on Linux
The text 68f76ce8bff6, as Debian 3.0 (Woody) shipped it.
reorder the generalized Schur decomposition of a complex matrix pair (A,B), using an unitary equivalence transformation (A, B) := Q * (A, B) * Z', so that the diagonal block of (A, B) with row index IFST is moved to row ILST