ir (A,C) to one of the following observer Hessenberg
  forms:

                             N
                    |*  . . . . . .  *|
                    |.               .|
                    |.               .|
                    |.               .| N
                    |*               .|
         |U'AU|     |   .            .|
         |----|  =  |     .          .|
         |CU  |     |       * . . .  *|
                    -------------------
                    |         * . .  *|
                