CKXTND    CKXTND    CKXTND    CKXTND    CKXTND    CKXTND    CKXTND    
          **********************************************
                    **************************
                              ******

SUBROUTINE CKXTND (NDIM, NPTS, XSTR, XEND, X, F, IFLAG)
Ensure that XSTR <= X(N) <= XEND.
NPTS may be increased to add  XSTR < X(1) or XEND > X(NPTS).
NPTS may be decreased to drop X(N) < XSTR or X(N) > XEND.
If NDIM does not allow adding a new endpoint,
CKXTND replaces the endpoint and sets IFLAG=1 if new XSTR,
                                      IFLAG=2 if new XEND.