*-- Author :  R. Henderson
      SUBROUTINE FPRCHI(PSEG,PERSEG,RSEG,RERSEG,Z,PROBCH)
**: FPRCHI.......SM. Bug fix. Protect against small Chisq for PROB.                                           
**----------------------------------------------------------------------                                      
C                                                                                                             
C---  Routine calculates the probability of PSEG and RSEG                                                     
C---  being the same vector at Z given that PERSEG is the covariance                                          
C---  matrix of PSEG both calculated at at Z=0                                                                
C---  (these are the quantities supplied by FPFYUV)                                                           
C---  and RERSEG is the covariance matrix of RSEG at Z                                                        
C                                                                                                             
C---  The probability fron chisquare is returned as PROB                                                      
C                                                                                                             
C---                                                                                                          
C                                                                                                             
C---  Construct the derivative matrix                                                                         
C                                                                                                             
      CALL VZERO(DERIV,16)                                                                             
C                                                                                                             
C---  Calculate X and Y at Z                                                                                  
C                                                                                                             
C                                                                                                             
C---  Propagate PERSEG FROM Z=0 TO Z                                                                          
C                                                                                                             
      CALL VZERO(WM,16)                                                                                
C---                                                                                                          
      CALL VZERO(COVAR,32)                                                                             
C---                                                                                                          
C      DO 111 I = 1,4                                                                                         
C      WRITE(6,1001)(COVAR(I,J),J=1,4)                                                                        
C 1001 FORMAT(1X,'COVAR',4e15.6)                                                                              
C  111 CONTINUE                                                                                               
C      WRITE(6,*)'------------------------------'                                                             
C                                                                                                             
C---  Add on the radial covariance matrix AT Z = Z                                                            
C                                                                                                             
C                                                                                                             
C---  Calculate weight matrix                                                                                 
C                                                                                                             
      CALL DINV(4,COVAR,4,WORK,IFAIL)                                                                  
C---                                                                                                          
C      DO 131 I = 1,4                                                                                         
C      WRITE(6,1003)(COVAR(I,J),J=1,4)                                                                        
C 1003 FORMAT(1X,'COVAR',4e15.6)                                                                              
C  131 CONTINUE                                                                                               
C      WRITE(6,*)'------------------------------'                                                             
C                                                                                                             
C---  Now calculate chisquare                                                                                 
C                                                                                                             
C                                                                                                             
C---  Find probability                                                                                        
C                                                                                                             
C      WRITE(6,*)PROBCH                                                                                       
C      WRITE(6,*)'------------------------------'                                                             
*