cover on INSERT
take level (even when NULL or nothing) as Z
wastewater_node on INSERT
take bottom level (even when NULL or nothing) as Z
reach and reach_point on INSERT
take level of reach_point (even when NULL or nothing) as Z of reach_point and reach
cover ON UPDATE
if level changed, take level as Z / if not: if Z changed, take Z as level
wastewater_node on UPDATE
if bottom_level changed, take bottom_level as Z / if not: if Z changed, take Z as bottom_level
reach and reach_point on UPDATE
if level of reach_point changed, take level as Z of reach_point and reach / if not: if Z changed, take Z as level of reach_point