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      SUBROUTINE DROTMG(DD1,DD2,DX1,DY1,DPARAM)
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*     .. Scalar Arguments ..
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      DOUBLE PRECISION DD1,DD2,DX1,DY1
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*     ..
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*     .. Array Arguments ..
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      DOUBLE PRECISION DPARAM(5)
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*     ..
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*
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*  Purpose
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*  =======
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*
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*     CONSTRUCT THE MODIFIED GIVENS TRANSFORMATION MATRIX H WHICH ZEROS
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*     THE SECOND COMPONENT OF THE 2-VECTOR  (DSQRT(DD1)*DX1,DSQRT(DD2)*
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*     DY2)**T.
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*     WITH DPARAM(1)=DFLAG, H HAS ONE OF THE FOLLOWING FORMS..
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*
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*     DFLAG=-1.D0     DFLAG=0.D0        DFLAG=1.D0     DFLAG=-2.D0
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*
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*       (DH11  DH12)    (1.D0  DH12)    (DH11  1.D0)    (1.D0  0.D0)
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*     H=(          )    (          )    (          )    (          )
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*       (DH21  DH22),   (DH21  1.D0),   (-1.D0 DH22),   (0.D0  1.D0).
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*     LOCATIONS 2-4 OF DPARAM CONTAIN DH11, DH21, DH12, AND DH22
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*     RESPECTIVELY. (VALUES OF 1.D0, -1.D0, OR 0.D0 IMPLIED BY THE
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*     VALUE OF DPARAM(1) ARE NOT STORED IN DPARAM.)
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*
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*     THE VALUES OF GAMSQ AND RGAMSQ SET IN THE DATA STATEMENT MAY BE
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*     INEXACT.  THIS IS OK AS THEY ARE ONLY USED FOR TESTING THE SIZE
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*     OF DD1 AND DD2.  ALL ACTUAL SCALING OF DATA IS DONE USING GAM.
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*
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*
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*  Arguments
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*  =========
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*
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*  DD1    (input/output) DOUBLE PRECISION
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*
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*  DD2    (input/output) DOUBLE PRECISION
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*
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*  DX1    (input/output) DOUBLE PRECISION
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*
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*  DY1    (input) DOUBLE PRECISION
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*
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*  DPARAM (input/output)  DOUBLE PRECISION array, dimension 5
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*     DPARAM(1)=DFLAG
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*     DPARAM(2)=DH11
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*     DPARAM(3)=DH21
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*     DPARAM(4)=DH12
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*     DPARAM(5)=DH22
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*
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*  =====================================================================
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*
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*     .. Local Scalars ..
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      DOUBLE PRECISION DFLAG,DH11,DH12,DH21,DH22,DP1,DP2,DQ1,DQ2,DTEMP,
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     +                 DU,GAM,GAMSQ,ONE,RGAMSQ,TWO,ZERO
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      INTEGER IGO
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*     ..
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*     .. Intrinsic Functions ..
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      INTRINSIC DABS
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*     ..
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*     .. Data statements ..
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*
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      DATA ZERO,ONE,TWO/0.D0,1.D0,2.D0/
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      DATA GAM,GAMSQ,RGAMSQ/4096.D0,16777216.D0,5.9604645D-8/
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*     ..
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      IF (.NOT.DD1.LT.ZERO) GO TO 10
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*       GO ZERO-H-D-AND-DX1..
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      GO TO 60
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   10 CONTINUE
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*     CASE-DD1-NONNEGATIVE
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      DP2 = DD2*DY1
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      IF (.NOT.DP2.EQ.ZERO) GO TO 20
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      DFLAG = -TWO
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      GO TO 260
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*     REGULAR-CASE..
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   20 CONTINUE
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      DP1 = DD1*DX1
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      DQ2 = DP2*DY1
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      DQ1 = DP1*DX1
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*
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      IF (.NOT.DABS(DQ1).GT.DABS(DQ2)) GO TO 40
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      DH21 = -DY1/DX1
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      DH12 = DP2/DP1
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*
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      DU = ONE - DH12*DH21
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*
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      IF (.NOT.DU.LE.ZERO) GO TO 30
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*         GO ZERO-H-D-AND-DX1..
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      GO TO 60
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   30 CONTINUE
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      DFLAG = ZERO
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      DD1 = DD1/DU
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      DD2 = DD2/DU
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      DX1 = DX1*DU
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*         GO SCALE-CHECK..
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      GO TO 100
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   40 CONTINUE
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      IF (.NOT.DQ2.LT.ZERO) GO TO 50
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*         GO ZERO-H-D-AND-DX1..
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      GO TO 60
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   50 CONTINUE
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      DFLAG = ONE
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      DH11 = DP1/DP2
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      DH22 = DX1/DY1
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      DU = ONE + DH11*DH22
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      DTEMP = DD2/DU
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      DD2 = DD1/DU
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      DD1 = DTEMP
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      DX1 = DY1*DU
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*         GO SCALE-CHECK
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      GO TO 100
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*     PROCEDURE..ZERO-H-D-AND-DX1..
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   60 CONTINUE
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      DFLAG = -ONE
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      DH11 = ZERO
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      DH12 = ZERO
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      DH21 = ZERO
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      DH22 = ZERO
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*
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      DD1 = ZERO
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      DD2 = ZERO
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      DX1 = ZERO
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*         RETURN..
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      GO TO 220
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*     PROCEDURE..FIX-H..
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   70 CONTINUE
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      IF (.NOT.DFLAG.GE.ZERO) GO TO 90
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*
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      IF (.NOT.DFLAG.EQ.ZERO) GO TO 80
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      DH11 = ONE
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      DH22 = ONE
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      DFLAG = -ONE
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      GO TO 90
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   80 CONTINUE
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      DH21 = -ONE
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      DH12 = ONE
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      DFLAG = -ONE
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   90 CONTINUE
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      GO TO IGO(120,150,180,210)
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*     PROCEDURE..SCALE-CHECK
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  100 CONTINUE
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  110 CONTINUE
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      IF (.NOT.DD1.LE.RGAMSQ) GO TO 130
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      IF (DD1.EQ.ZERO) GO TO 160
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      ASSIGN 120 TO IGO
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*              FIX-H..
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      GO TO 70
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  120 CONTINUE
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      DD1 = DD1*GAM**2
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      DX1 = DX1/GAM
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      DH11 = DH11/GAM
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      DH12 = DH12/GAM
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      GO TO 110
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  130 CONTINUE
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  140 CONTINUE
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      IF (.NOT.DD1.GE.GAMSQ) GO TO 160
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      ASSIGN 150 TO IGO
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*              FIX-H..
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      GO TO 70
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  150 CONTINUE
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      DD1 = DD1/GAM**2
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      DX1 = DX1*GAM
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      DH11 = DH11*GAM
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      DH12 = DH12*GAM
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      GO TO 140
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  160 CONTINUE
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  170 CONTINUE
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      IF (.NOT.DABS(DD2).LE.RGAMSQ) GO TO 190
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      IF (DD2.EQ.ZERO) GO TO 220
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      ASSIGN 180 TO IGO
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*              FIX-H..
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      GO TO 70
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  180 CONTINUE
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      DD2 = DD2*GAM**2
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      DH21 = DH21/GAM
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      DH22 = DH22/GAM
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      GO TO 170
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  190 CONTINUE
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  200 CONTINUE
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      IF (.NOT.DABS(DD2).GE.GAMSQ) GO TO 220
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      ASSIGN 210 TO IGO
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*              FIX-H..
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      GO TO 70
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  210 CONTINUE
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      DD2 = DD2/GAM**2
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      DH21 = DH21*GAM
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      DH22 = DH22*GAM
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      GO TO 200
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  220 CONTINUE
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      IF (DFLAG) 250,230,240
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  230 CONTINUE
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      DPARAM(3) = DH21
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      DPARAM(4) = DH12
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      GO TO 260
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  240 CONTINUE
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      DPARAM(2) = DH11
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      DPARAM(5) = DH22
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      GO TO 260
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  250 CONTINUE
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      DPARAM(2) = DH11
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      DPARAM(3) = DH21
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      DPARAM(4) = DH12
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      DPARAM(5) = DH22
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  260 CONTINUE
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      DPARAM(1) = DFLAG
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      RETURN
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      END