TODO

1. hrd.ijs: gghrdxxx: implement blocked generalized
   Hessenberg reduction xGGHD3 [1]

2. (lacn2,gesc2,latdf,getc2,tgsy2,tgsyl,tgex2,tgexc,tgsen) ->
   gges3

3. (gges3,tgsyl) -> [2]

4. improve *NB and *NX constants for optimal performance

5. exp.ijs: implement updated algo [3]

6. Sylvester equation solvers:
   [generalized] {continuous,discrete}-time

7. Lyapunov equation solvers:
   [generalized] {continuous,discrete}-time
   for {solution X,Cholesky factor U}

8. Algebraic Riccati equation solvers:
   [generalized] {continuous,discrete}-time

9. replace pf (RRQR) by qp (xGEQP3) [4]

10. geqp3 -> gelsy

11. (getc2,geqp3) -> a matrix canonization algorithm [5]

12. rewrite loops via Fold (F.. F.: F. F:. F:: F:)

References:
[1] Bo Kågström, Daniel Kressner, Enrique S. Quintana-Ortí,
    and Gregorio Quintana-Ortí. Blocked Algorithms for the
    Reduction to Hessenberg-Triangular Form Revisited.
    February 2008. LAPACK Working Note 198
    http://www.netlib.org/lapack/lawns/downloads/

[2] Gerdin, Markus. Computation of a canonical form for
    linear differential-algebraic equations. Department of
    Electrical Engineering, Linköping University, 2004.
    LiTH-ISY-R-2602

[3] Awad H. Al-Mohy, Nicholas J. Higham. A New Scaling and
    Squaring Algorithm for the Matrix Exponential. SIAM J.
    Matrix Analysis Applications, Vol. 31, No. 3, pp.
    970-989, 2009.

[4] Rank Revealing QR factorization (RRQR)
    LAPACK/ScaLAPACK Development forum. 2012-02-06.
    http://icl.cs.utk.edu/lapack-forum/viewtopic.php?f=5&t=2469#p8162

[5] Volkov, V.G., Dem’yanov, D.N. Application of Matrix
    Decompositions for Matrix Canonization. Comput. Math. and
    Math. Phys. 59, 1759–1770 (2019).
    https://doi.org/10.1134/S0965542519110149