What is the apportionment of diya if the deaths are caused by falling upon one another, absent external factors in the well?

Primary text

If their deaths are caused by falling upon one another, the fourth person's blood money is nullified because no other party acted upon him; he perished by his own action. The diya of the third person is incumbent upon the fourth due to the fourth's impact upon him. The diya of the second person is split equally between the fourth and the third. The diya of the first person is incumbent upon the three subsequent parties in equal thirds.