*-- Author :  R. Henderson
      SUBROUTINE FPCFIT(IP,Y,W,SLOPE,ZERO,COVSLZ,CHISQ,PBCHI)
**: FPCFIT.......SM. Bug fix. Protect against small Chisq for PROB.                                           
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C-------------------------------------------------------------                                                
C                                                                                                             
C---  Fit four points y(i) with weights w(i) at positions z(i)                                                
C---  Returns SLOPE (dy/dz) ZERO (y(0))                                                                       
C---          COVSLZ(2,2) their correlation matrix                                                            
C---  and     CHISQ and probability from chisquare pbchi                                                      
C                                                                                                             
C-------------------------------------------------------------                                                
C---                                                                                                          
*KEEP,FPLGEO.                                                                                                 
C---                                                                                                          
C---                                                                                                          
*KEND.                                                                                                        
C---                                                                                                          
C                                                                                                             
C---  Section of code varies with w(i)                                                                        
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C                                                                                                             
C---  return slope and constant unphysical if det is 0                                                        
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C---  calculate error matrix                                                                                  
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C---  Initialization per fit                                                                                  
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C                                                                                                             
C---  Calculate required sums                                                                                 
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C                                                                                                             
C---  Calculate slopes and data zeros                                                                         
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C                                                                                                             
C---  Calculate chisquare                                                                                     
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C                                                                                                             
C---  Calculate probability from chisquare                                                                    
C                                                                                                             
*     Fix for v. small Chisq...                                                                               
*