DLAEXC swaps adjacent diagonal blocks T11 and T22 of order 1 or 2 in
 an upper quasi-triangular matrix T by an orthogonal similarity
 transformation.

 T must be in Schur canonical form, that is, block upper triangular
 with 1-by-1 and 2-by-2 diagonal blocks; each 2-by-2 diagonal block
 has its diagonal elemnts equal and its off-diagonal elements of
 opposite sign.

          WANTQ is LOGICAL
          = .TRUE. : accumulate the transformation in the matrix Q;
          = .FALSE.: do not accumulate the transformation.

          N is INTEGER
          The order of the matrix T. N >= 0.

          T is DOUBLE PRECISION array, dimension (LDT,N)
          On entry, the upper quasi-triangular matrix T, in Schur
          canonical form.
          On exit, the updated matrix T, again in Schur canonical form.

          LDT is INTEGER
          The leading dimension of the array T. LDT >= max(1,N).

          Q is DOUBLE PRECISION array, dimension (LDQ,N)
          On entry, if WANTQ is .TRUE., the orthogonal matrix Q.
          On exit, if WANTQ is .TRUE., the updated matrix Q.
          If WANTQ is .FALSE., Q is not referenced.

          LDQ is INTEGER
          The leading dimension of the array Q.
          LDQ >= 1; and if WANTQ is .TRUE., LDQ >= N.

          J1 is INTEGER
          The index of the first row of the first block T11.

          N1 is INTEGER
          The order of the first block T11. N1 = 0, 1 or 2.

          N2 is INTEGER
          The order of the second block T22. N2 = 0, 1 or 2.

          WORK is DOUBLE PRECISION array, dimension (N)

          INFO is INTEGER
          = 0: successful exit
          = 1: the transformed matrix T would be too far from Schur
               form; the blocks are not swapped and T and Q are
               unchanged.