anchor.diff:      Q(1:nn) = (h_ray(1:nn) * radearth - hterr(1:nn)) &
             & / (raylength * t(1:nn))
       attn = th(minval(Q(1:nn)) / real(0.001,fp))
  -  end subroutine checkline
  +  end subroutine checkray
   
     !-----------------------------------------------------------------------------
   
  
positive.code: 
    !-----------------------------------------------------------------------------
  
    pure subroutine checkline(maph, gth, lat1, lng1, h1, lat2, lng2, h2, attn)
      ! altitude map and geotransform
      real, dimension(:,:), intent(in) :: maph
      type(geotransform), intent(in) :: gth
  
negative.code:           & JJ(:,1), JJ(:,2), JJ(:,3))
  
        check_lines_all_heights: do k = 1,nh
          call checkray(maph, gth,                                &
                & src(i,j) % lat, src(i,j) % lng, src(i,j) % h,   &
                & lat,            lng,            hsct(k),        &
                & JJ(k,4))
  