SUBROUTINE DROTM(N,SX,INCX,SY,INCY,SPARAM)
  
DROTM applies the modified Givens rotation H to the 2 by N matrix

  ( X(1) ... X(N) )
  ( Y(1) ... Y(N) )
 
H takes one of the following forms, depending on the value of SPARAM:
 
SPARAM(1) = -2.0
H11 = 1.0        H12 = 0.0
H21 = 0.0        H22 = 1.0
 
SPARAM(1) = -1.0
H11 = SPARAM(2)  H12 = SPARAM(4)
H21 = SPARAM(3)  H22 = SPARAM(5)
 
SPARAM(1) = 0.0
H11 = 1.0        H12 = SPARAM(4)
H21 = SPARAM(3)  H22 = 1.0
 
SPARAM(1) = 1.0
H11 = SPARAM(2)  H12 = 1.0
H21 = -1.0       H22 = SPARAM(5)
 
N      Input, INTEGER N, length of vectors X and Y.
 
SX     Input/output, DOUBLE PRECISION SX(N).
       DROTM replaces X(I) with H11*X(I)+H12*Y(I) for I=1,...,N.
       X(I) and Y(I) refer to specific elements of SX and SY.
       The H components refer to the rotation defined by SPARAM.
 
INCX   Input, INTEGER INCX, displacement between elements of SX.
       X(I) is defined to be
       SX(1+(I-1)*INCX) if INCX >= 0  or
       SX(1+(I-N)*INCX) if INCX < 0.
 
SY     Input/output, DOUBLE PRECISION SY(N).
       DROTM replaces Y(I) with H21*X(I)+H22*Y(I) for I=1,...,N.
       X(I) and Y(I) refer to specific elements of SX and SY.
       The H components refer to the rotation defined by SPARAM.
 
INCY   Input, INTEGER INCY, displacement between elements of SY.
       Y(I) is defined to be
       SY(1+(I-1)*INCY) if INCY >= 0  or
       SY(1+(I-N)*INCY) if INCY < 0.
 
SPARAM Input, DOUBLE PRECISION SPARAM(5), defines the rotation matrix H.
       See remarks above.