keep rows with most recent date

[Richard]

In my database, each row corresponds to a new purchase or ammended purchase.

The key fields are:
purchase number
purchase linenumber
date
description
amount

When a purchase is fist entered, it is given a purchase number and
linenumber which never vary, plus a date, description and amount. Later, if
someone changes their mind, a new entry will be made with the same purchase
number and linenumber, but the description or amount will be different.

I want to keep only the rows that have a given purchase number and
linenumber but with the most recent date.