 that each
  cluster of generalized eigenvalues, defined as specified in the
  definition of TOL, appears in adjacent elements. The elements for
  each cluster are merged together, and the procedure described
  above is applied to the larger blocks. Using the option SORT = 'S'
  will usually provide better efficiency than the standard option
  (SORT = 'N'), proposed in [2], because there could be no or few
  unsuccessful attempts to compute individual transformation
  matrices X and Y of the form (1). Howev