Query Help - Sequential Numbers

I have a query which returns information about transactions similar to this:

Select account, trans_code, quantity
from ledger_table

This returns something like:

acct trans_code quantity
----- ---------- ------------
2 2 1000
2 3 500
2 3 300
3 2 100
3 2 500
etc.

What I need to do is add a lot number for each acct/trans code type. This is merely a sequential number for the transaction. This changes the output as follows:

acct trans_code quantity lot
----- ---------- ------------ ---
2 2 1000 1
2 3 500 1
2 3 300 2
3 2 100 1
3 2 500 2
3 3 1000 3
3 5 200 1
etc.

The lot number is reset for each grouping.

Does anyone have a way to do this outside of a cursor or temp table?

TIA, any help greatly appreciated.