Fist bitcoins in, First out.
Will this assumption allow individual bitcoin tracking possible?
example,
Addresses A, B and C each has 50 BTC
Transactions,
tx1 - A > D - 10 BTC (D - 10 BTC)
tx2 - B > D - 15 BTC (D - 25 BTC)
tx3 - C > D - 20 BTC (D - 45 BTC)
tx4 - D > E - 20 BTC (E - 20 BTC, D - 25 BTC)
Now,
D has none of BTC received from A, 5 BTC from B and 20 BTC from C
E, fist 10 BTC was received from A, next 10 BTC was received from B