contents of A on exit are illustrated by the following example
    with n = 7, k = 3 and nb = 2:

    @verbatim
       ( a   a   a   a   a )
       ( a   a   a   a   a )
       ( a   a   a   a   a )
       ( h   h   a   a   a )
       ( v1  h   a   a   a )
       ( v1  v2  a   a   a )
       ( v1  v2  a   a   a )
    @endverbatim

    where "a" denotes an element of the original matrix A, h denotes a
    modified element of the upper Hessenberg matrix H, and vi denotes an
    element of the vector defining 