```      SUBROUTINE CTREXC( COMPQ, N, T, LDT, Q, LDQ, IFST, ILST, INFO )
*
*  -- LAPACK routine (version 3.1) --
*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
*     November 2006
*
*     .. Scalar Arguments ..
CHARACTER          COMPQ
INTEGER            IFST, ILST, INFO, LDQ, LDT, N
*     ..
*     .. Array Arguments ..
COMPLEX            Q( LDQ, * ), T( LDT, * )
*     ..
*
*  Purpose
*  =======
*
*  CTREXC reorders the Schur factorization of a complex matrix
*  A = Q*T*Q**H, so that the diagonal element of T with row index IFST
*  is moved to row ILST.
*
*  The Schur form T is reordered by a unitary similarity transformation
*  Z**H*T*Z, and optionally the matrix Q of Schur vectors is updated by
*  postmultplying it with Z.
*
*  Arguments
*  =========
*
*  COMPQ   (input) CHARACTER*1
*          = 'V':  update the matrix Q of Schur vectors;
*          = 'N':  do not update Q.
*
*  N       (input) INTEGER
*          The order of the matrix T. N >= 0.
*
*  T       (input/output) COMPLEX array, dimension (LDT,N)
*          On entry, the upper triangular matrix T.
*          On exit, the reordered upper triangular matrix.
*
*  LDT     (input) INTEGER
*          The leading dimension of the array T. LDT >= max(1,N).
*
*  Q       (input/output) COMPLEX array, dimension (LDQ,N)
*          On entry, if COMPQ = 'V', the matrix Q of Schur vectors.
*          On exit, if COMPQ = 'V', Q has been postmultiplied by the
*          unitary transformation matrix Z which reorders T.
*          If COMPQ = 'N', Q is not referenced.
*
*  LDQ     (input) INTEGER
*          The leading dimension of the array Q.  LDQ >= max(1,N).
*
*  IFST    (input) INTEGER
*  ILST    (input) INTEGER
*          Specify the reordering of the diagonal elements of T:
*          The element with row index IFST is moved to row ILST by a
*          sequence of transpositions between adjacent elements.
*          1 <= IFST <= N; 1 <= ILST <= N.
*
*  INFO    (output) INTEGER
*          = 0:  successful exit
*          < 0:  if INFO = -i, the i-th argument had an illegal value
*
*  =====================================================================
*
*     .. Local Scalars ..
LOGICAL            WANTQ
INTEGER            K, M1, M2, M3
REAL               CS
COMPLEX            SN, T11, T22, TEMP
*     ..
*     .. External Functions ..
LOGICAL            LSAME
EXTERNAL           LSAME
*     ..
*     .. External Subroutines ..
EXTERNAL           CLARTG, CROT, XERBLA
*     ..
*     .. Intrinsic Functions ..
INTRINSIC          CONJG, MAX
*     ..
*     .. Executable Statements ..
*
*     Decode and test the input parameters.
*
INFO = 0
WANTQ = LSAME( COMPQ, 'V' )
IF( .NOT.LSAME( COMPQ, 'N' ) .AND. .NOT.WANTQ ) THEN
INFO = -1
ELSE IF( N.LT.0 ) THEN
INFO = -2
ELSE IF( LDT.LT.MAX( 1, N ) ) THEN
INFO = -4
ELSE IF( LDQ.LT.1 .OR. ( WANTQ .AND. LDQ.LT.MAX( 1, N ) ) ) THEN
INFO = -6
ELSE IF( IFST.LT.1 .OR. IFST.GT.N ) THEN
INFO = -7
ELSE IF( ILST.LT.1 .OR. ILST.GT.N ) THEN
INFO = -8
END IF
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'CTREXC', -INFO )
RETURN
END IF
*
*     Quick return if possible
*
IF( N.EQ.1 .OR. IFST.EQ.ILST )
\$   RETURN
*
IF( IFST.LT.ILST ) THEN
*
*        Move the IFST-th diagonal element forward down the diagonal.
*
M1 = 0
M2 = -1
M3 = 1
ELSE
*
*        Move the IFST-th diagonal element backward up the diagonal.
*
M1 = -1
M2 = 0
M3 = -1
END IF
*
DO 10 K = IFST + M1, ILST + M2, M3
*
*        Interchange the k-th and (k+1)-th diagonal elements.
*
T11 = T( K, K )
T22 = T( K+1, K+1 )
*
*        Determine the transformation to perform the interchange.
*
CALL CLARTG( T( K, K+1 ), T22-T11, CS, SN, TEMP )
*
*        Apply transformation to the matrix T.
*
IF( K+2.LE.N )
\$      CALL CROT( N-K-1, T( K, K+2 ), LDT, T( K+1, K+2 ), LDT, CS,
\$                 SN )
CALL CROT( K-1, T( 1, K ), 1, T( 1, K+1 ), 1, CS, CONJG( SN ) )
*
T( K, K ) = T22
T( K+1, K+1 ) = T11
*
IF( WANTQ ) THEN
*
*           Accumulate transformation in the matrix Q.
*
CALL CROT( N, Q( 1, K ), 1, Q( 1, K+1 ), 1, CS,
\$                 CONJG( SN ) )
END IF
*
10 CONTINUE
*
RETURN
*
*     End of CTREXC
*
END

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