ADAS Subroutine b2ngas

       SUBROUTINE B2NGAS(X,DX,FORM,IFORMS,IENDS)                        
C
       IMPLICIT REAL*8(A-H,O-Z)                                         
C-----------------------------------------------------------------------
C
C  ****************** FORTRAN77 SUBROUTINE: B2NGAS *********************
C
C PURPOSE:
C  SUBROUTINE INITIALISES COMMON ARRAYS REQUIRED FOR SPLINING WITH
C  SMOOTH FITTING TO AN ASYMPTOTIC FORM                                 
C
C
C  USES LABELLED COMMON /SPL3/
C
C  IF IENDS=1,MATCHING IS AT FIRST KNOT(GIVEN BY X)                     
C          =2,MATCHING IS AT  LAST KNOT(GIVEN BY X)                     
C  ASYMPTOTIC FORMS ARE GIVEN IN THE EXTERNAL FUNCTION FORM(I,X)        
C  WHERE I=4*IFORMS-5+2*IENDS POINTS TO FIRST PART OF ASYMP. FORM       
C         =4*IFORMS-4+2*IENDS POINTS TO SECOND PART OF ASYMP. FORM      
C  THUS A SERIES OF ASYMPTOTIC FORMS MAY BE PRESENT IN FORM             
C
C  INPUT
C      COMMON /SPL3/ PROVIDES INPUT IN VECTOR IEND WHICH SPECIFIES
C                     CHOICE OF END CONDITION AT FIRST IEND(1) OR LAST
C                     IEND(2) KNOT OF SPLINE
C      X=X-VALUE OF END POINT
C      DX=DISPLACEMENT FROM X-VALUE FOR DERIVATIVE EVALUATION
C      FORM=EXTERNAL FUNCTION SPECIFYING ASYMPTOTIC FORMS
C      IFORMS=SELECTED FORM
C      IENDS=1,MATCHING IS AT FIRST KNOT(GIVEN BY X)                    
C           =2,MATCHING IS AT  LAST KNOT(GIVEN BY X)                    
C  OUTPUT
C      COMMON /SPL3/ IS SET BY THIS ROUTINE
C
C NOTES: THIS ROUTINE IS NOT YET PROPERLY ANNOTATED
C
C UNIX-IDL PORT:
C
C VERSION: 1.1                          DATE: 06-03-96
C MODIFIED: TIM HAMMOND (TESSELLA SUPPORT SERVICES PLC)
C               - PUT UNDER S.C.C.S. CONTROL
C
C-----------------------------------------------------------------------
C
       COMMON /SPL3/IEND(2),G(2),AB(4),PQ(12),ABRY(40)                  
       IF(IENDS.EQ.1.AND.IEND(1).EQ.4)GO TO 5                           
       IF(IENDS.EQ.2.AND.IEND(2).EQ.4)GO TO 5                           
    3  RETURN                                                           
    5  I=4*IFORMS-5+2*IENDS                                             
       J=6*IENDS-5                                                      
       IC=0                                                             
       DX1=1.0D0/DX                                                     
   10  PQ(J)=FORM(I,X)                                                  
       T1=FORM(I,X+DX)                                                  
       T2=FORM(I,X-DX)                                                  
       PQ(J+1)=0.5D0*DX1*(T1-T2)                                        
       PQ(J+2)=DX1*DX1*(T1-2.0D0*PQ(J)+T2)                              
       IC=IC+1                                                          
       IF(IC.GT.1)RETURN                                                
       I=I+1                                                            
       J=J+3                                                            
       GO TO 10                                                         
      END                                                              
      INTEGER             IENDS,       IFORMS
      REAL*8              DX,          X