SUBROUTINE DSWAP(N,SX,INCX,SY,INCY)
 
exchanges two real vectors SX and SY.
 
N      Input, INTEGER N, number of elements in vectors.
 
SX     Input/output, DOUBLE PRECISION SX(N).  On output, the contents of
       SX and SY have been interchanged.
 
INCX   Input, INTEGER INCX, increment between elements of SX.  For contiguous
       elements, INCX = 1.
 
SY     Input/output, DOUBLE PRECISION SY(N), the other vector to interchange.
 
INCY   Input, INTEGER INCY, increment between elements of SY.  For contiguous
       elements, INCY = 1.