(if (not background-tm) (error "No background tilemap given"))
(if (not foreground-tm) (error "No foreground tilemap given"))
(if (not objects) (error "No objects list given"))
(if (not background-tm) (error "No background tilemap given"))
(if (not foreground-tm) (error "No foreground tilemap given"))
(if (not objects) (error "No objects list given"))