SUBROUTINE SROTMG(SD1,SD2,SX1,SY1,SPARAM)
  
SROTMG constructs a modified Givens rotation H and updates the
scale factors SD1 and SD2 which zero SY1. The transformed value of
SD1 replaces SD1, i.e.

On input,  SW1  =  SQRT(SD1)*SX1, SZ1  =  SQRT(SD2)*SY1.

On output, ( C  S ) (SW1) = (C*SW1 + S*SZ1) = (SQRT(SD1)*SX1)
           (-S  C ) (SZ1) = (      0      ) = (          0  )
where C and S define a Givens rotation.
 
H takes the form:
 
SPARAM(1) = -2.0
SPARAM(2) = Unchanged   SPARAM(4) = Unchanged
SPARAM(3) = Unchanged   SPARAM(5) = Unchanged
 
SPARAM(1) = -1.0
SPARAM(2) = H11    SPARAM(4) = H12
SPARAM(3) = H21    SPARAM(5) = H22
 
SPARAM(1) = 0.0
SPARAM(2) = Unchanged   SPARAM(4) = H12
SPARAM(3) = H21         SPARAM(5) = Unchanged
 
SPARAM(1) = 1.0
SPARAM(2) = H11    SPARAM(4) = Unchanged
  
SD1    Input/output, REAL SD1, scale factor.
       On input, SD1 contains the first scale factor.  On
       output, SD1 contains the updated scale factor.
 
SD2    Input/output, REAL SD2, scale factor.
       On input, SD2 contains the second scale factor.  On
       output, SD2 contains the updated scale factor.
 
SX1    Input/output, REAL SX1, on input contains the first component of the 
       vector to be rotated and output SX1 contains the rotated value
       of the first component.
 
SY1    Input, REAL SY1, on input, second component of the vector to be 
       rotated.  Since this component is zeroed by the rotation, it is
       left unchanged in storage.
 
SPARAM Input/output, REAL SPARAM(5), defines the rotation matrix H.
       See remarks above.