C===============================================================================
      SUBROUTINE CTVMMF (PRINT, NM, VXI)
C===============================================================================
C  Derivative contributions for the CTVM mass constraint fitting
C--Input parameter
C   Nm           The mass constraint index
C-------------------------------------------------------------------------------
        IMPLICIT NONE
#include <Stntuple/fit/ctvmft.inc>
      INTEGER PRINT
      INTEGER NM
      DOUBLE PRECISION VXI(MAXDIM)
      DOUBLE PRECISION SUM
      INTEGER I,J, Nt,Nv, LmF,NvF,NtF
      REAL    C
C-------------------------------------------------------------------------------
      LmF = MOFF + Nm
C  Difference in m**2 of tracks and constraint / 2
      SUM = SQRT(McnP4(1,Nm)**2 + McnP4(2,Nm)**2 + McnP4(3,Nm)**2)
      SUM = (McnP4(4,Nm) + SUM) * (McnP4(4,Nm) - SUM)
      SUM = DSQRT(SUM)
      VXI(LmF) = 0.5 * (CMASS(Nm)+SUM) * (CMASS(Nm)-SUM)
C  Loop over tracks contributing to this mass constraint
      DO 50 Nv=1,NVERTX
      NvF = VOFF(Nv)
      DO 40 NT=1,NTRACK
      IF (.NOT.TrkVtx(Nt,Nv)) GO TO 40
      IF (.NOT.TrkMcn(Nt,Nm)) GO TO 40
C Index into matrix equations for track NT
        NtF = TOFF(Nt)               
C  dM**2/dc, dM**2/dphi, dM**2/dcot(theta)
        C   = PAR0(1,Nt) + PARDIF(1,Nt)
        SUM = -McnP4(4,Nm)*TrkP4(Nt,6)**2/TrkP4(Nt,4)
        SUM = (SUM + McnP4(3,Nm)*TrkP4(Nt,3)) / C
        SUM = SUM - McnP4(1,Nm)*DDA(Nt,1) - McnP4(2,Nm)*DDA(Nt,5)
        VMAT(NtF+1,LmF) = SUM
        SUM =-McnP4(1,Nm)*DDA(Nt,2) - McnP4(2,Nm)*DDA(Nt,6)
        VMAT(NtF+2,LmF) = SUM
        SUM = (McnP4(4,Nm)*TrkP4(Nt,3) / TrkP4(Nt,4)-McnP4(3,Nm))
        SUM = TrkP4(Nt,5) * SUM
        VMAT(NtF+3,LmF) = SUM
C  dM**2/dXsv, dM**2/dYsv
        VMAT(NvF+1,LmF) = VMAT(NvF+1,LmF)
     &              - McnP4(1,Nm)*DDA(Nt,3)-McnP4(2,Nm)*DDA(Nt,7)
        VMAT(NvF+2,LmF) = VMAT(NvF+2,LmF)
     &              - McnP4(1,Nm)*DDA(Nt,4)-McnP4(2,Nm)*DDA(Nt,8)
D     IF (PRINT.GT.0)  THEN
D     WRITE(PRINT,1045) NT, VXI(LmF)
D    @,     (VMAT(NvF+J,LmF),J=1,2),(VMAT(NtF+J,LmF),J=1,3)
D1045 FORMAT(/,' Mcn, Tk',I3, 1P,E11.3,2X,5E11.3)
D     END IF
   40 CONTINUE
C Continue loop on tracks
   50 CONTINUE              
C-------------------------------------------------------------------------------
C===Return to Caller:
C Symmetrize the derivative matrix
      DO I=1,MATDIM-1               
      DO J=I+1,MATDIM
         VMAT(J,I) = VMAT(I,J)
       ENDDO
      ENDDO
      RETURN
      END