Thank you, Nicholas, for the quote from Petty. Interesting to find him using
the concept of "years purchase", the same as modern Brits.  However, his
method of discounting future cash flows seems pretty crude, considering that
he published about a century after Napier had developed logarithms.

We can't accuse him of sacrificing sophisicated math for sentiment, for his
"Political Arithmetik" was stony-hearted enough to have inspired, so some
say, Swift's satire, "A Modest proposal".

Mason Gaffney