anchor.diff:      logical :: sane
       character(:), allocatable :: error_msg
   
  -    write(*,*) nx, nxl_ext, nxr_ext, nxl_vd, nxr_vd
  -
       sane = (nxl_ext .gt. 0) .and. (nxr_ext .gt. 0)
       if (.not. sane) then
          error_msg = "No external region defined in numerical grid; "// &
  
positive.code:     if (.not. sane) then
         error_msg = "Virtual detectors must be present in numerical grid; "// &
              "stopping abnormally."
         write(*,*) nxl_vd, nxr_vd
         call log_log_critical(error_msg, log_stderr)
         stop 0
      end if
  
negative.code:     end do
  
      call numerics_d1(this%phi_arr_r(:), this%p_arr_r(:), this%dx)
      this%j_arr_r(:) = this%mag_arr_r(:) / this%m * this%p_arr_r(:)
  
      call numerics_d2(log(this%mag_arr_r(:)), this%p_var_arr_r(:), &
           this%dx)
  