 A(L+1,L+1) )
  be the specified 2-by-2 diagonal block of matrix A.
  If the eigenvalues of A1 are complex, then they are computed and
  stored in E1 and E2, where the real part is stored in E1 and the
  positive imaginary part in E2. The 2-by-2 block is reduced if
  necessary to the standard form, such that A(L,L) = A(L+1,L+1), and
  A(L,L+1) and A(L+1,L) have oposite signs. If the eigenvalues are
  real, the 2-by-2 block is reduced to an upper triangular form such
  that ABS(A(L,L)) &gt;= ABS(A(L+1,L+1)).