Yahoo pipes: How to output the difference between two feeds?

Question

How can one output the difference between two feeds in Yahoo Pipes?

That is to say, I want all the posts from module A which do not occur in module B (filtered by title).

I've found how to get the set union using the Unique operator, but I can't find an obvious operator for getting the set difference.

Example

Let's say a newspaper website publishes two feeds, FrontPage.rss and Politics.rss, and that the most interesting items from the politics feed are also published on the front page feed among other items. We want to subscribe to both feeds separately, and we don't want receive politics articles in our front page feed, because we're already reading them elsewhere. We can subscribe directly to the politics feed, but for the front page feed we actually want all the items from FrontPage.rss except those which also occur in Politics.rss. That is, the set difference of FrontPage and Politics (also known as the relative complement of Politics in FrontPage).

Answer 1

In the operators section you should be able to use the "unique" module to filter based on a unique identifier such as a url.

Answer 2

Put both feeds through a "Union" module, then put the output through a "Unique" module.