module calcf_interface
interface
subroutine calcf(lat,nconstraints,nparams, chis, yfit,grid_half_width)
! compute the electric field over a grid (+-4.5 X +-4.5cm) using the grid map quadrupole element (which is ele(1)) in this lattice)
! then compute the electric field over the same grid using the multipole quadrupole element (ele(2) in this lattic)
! then take the sum of the squares of the difference of Ex and Ey over the entire grid.
! That sum is chis2

  use bmad

  implicit none

  type (lat_struct) lat
  integer nparams, nconstraints
  real(rp) chis 
  real(rp), allocatable:: yfit(:)
  real(rp), optional::grid_half_width  ! something less than 0.042 m 

 end subroutine
end interface

interface
subroutine calcf_dyda(lat,nconstraints,nparams,chis, yfit, dyda)
use bmad
implicit none
type(lat_struct) lat
  real(rp), allocatable :: yfit(:)
  real(rp), dimension(:,:) :: dyda
  real(rp) chis
  integer nconstraints, nparams

end subroutine
end interface
end module