However if you want to loop over an indeterminate-size list of URLs to find out when each page last changed, it seemed hard -- until I remembered tail recursion, which is pretty simple -- but I don't see many people talking about it. So I thought I'd write it up.
Arrange all the URLs in an array.
Write a routine that reads one URL.
When it completes, it recursively calls itself for the next item in the array.
When you fall off the array, return.