SUBROUTINE CHIOP(N,S2,ANU,NITER,JFIX,CHI)
C  This routine draws from N independent chi square distributions, subject
C  to the restriction that the realizations are monotonically nondescending.
C  One of the "chi-squares" is fixed at a preset value.
C  Inputs:
C  N         Number of distributions
C  S2(Nx1)   Constant parameters for distributions
C  ANU(Nx1)  Degrees of freedom parameters for distributions
C  NITER     Number of Gibbs iterations to perform
C  JFIX      Value of (pre-)fixed precision
C  Input/outputs:
C  CHI       Random vector
      IMPLICIT REAL*8 (A-H,O-Z)
      DIMENSION S2(N),ANU(N),CHI(N)
      N1=N-1
      DO 20 ITER=1,NITER
      IF(JFIX.NE.1)CHI(1)=XCHIT(S2(1),ANU(1),0.0D0,CHI(2),.FALSE.)
      IF(N.EQ.2)GO TO 20
      DO 10 I=2,N1
   10 IF(I.NE.JFIX)CHI(I)=XCHIT(S2(I),ANU(I),CHI(I-1),CHI(I+1),.FALSE.)
   20 IF(N.NE.JFIX)CHI(N)=XCHIT(S2(N),ANU(N),CHI(N1),-1.0D0,.TRUE.)
      RETURN
      END