! Subroutine dx_dvcros7_CALC (LAT, VC7_DIFF)
!
! Subroutine to calculate the dependence of orbit at detectors 0E and 0W
! on vcrosing 7. The group element raw_vcrosing_7 must be defined
!
!
! Input:
!     LAT      -- lat_struct: Lat
!
! Output:
!-    vc7_diff(0:lat%n_ele_track)  -- Coord_struct
!                        Difference between closed orbits with vcrosing7
!                           +- dk

subroutine dx_dvcros7_calc ( lat, vc7_diff)

  use bmadz_mod
  use bmadz_interface, dummy => dx_dvcros7_calc
  use bmad_interface

  implicit none

  type (lat_struct)  lat
  type (coord_struct), allocatable ::  vc7_diff(:)
  type (coord_struct), allocatable, save ::  coplus(:)
  type (coord_struct), allocatable, save ::  cominus(:)

  real(rp) :: vc7_command, dk, vec0(6) = 0
  integer ix_vc7/0/
  integer i, n_track

  n_track = lat%n_ele_track
  call reallocate_coord(coplus, lat)
  call reallocate_coord(cominus, lat)


!
! find raw_vcrosing_7

  i=0
   do while(ix_vc7 == 0)
   if(lat%ele(i)%name == 'RAW_VCROSING_7')ix_vc7=i
   i=i+1
     if(i > n_track) then
    print *,' DX_DVCROS7_CALC: cannot find RAW_VCROSING_7'
    stop
   endif
  enddo


  vc7_command = lat%ele(ix_vc7)%control%var(1)%value
  dk = 0.1

  lat%ele(ix_vc7)%control%var(1)%value = vc7_command*(1+dk)
  call lat_make_mat6(lat, ix_vc7)

  call closed_orbit_calc( lat, coplus, 4, +1 )

  lat%ele(ix_vc7)%control%var(1)%value = vc7_command*(1-dk)
  call lat_make_mat6(lat, ix_vc7)

  call init_coord (cominus(n_track), vec0, lat%ele(n_track), downstream_end$, antiparticle(lat%param%particle), -1)
  call closed_orbit_calc( lat, cominus, 4, -1 )

  lat%ele(ix_vc7)%control%var(1)%value = vc7_command
  call lat_make_mat6(lat, ix_vc7)
 

  do i = 1,n_track
   vc7_diff(i)%vec(:) = (coplus(i)%vec(:) - cominus(i)%vec(:))/(2*dk)
  end do

  return
end