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:     this%p_min = p_min
      this%p_max = p_max
  
      this%dt = dt
  
      this%semi_classical = sc
  