ReactiveCocoa/ReactiveCocoa — pull request #398 by iMartinKiss (2013-03-25 20:44:04+00:00)
Title: Add method `-includePrevious:` for combining latest and previous values

Discussed in #387, so here is the working implementation.

This allows you to access previous values of the stream. Example:

```objc
[[[RACAble(self, value)
includePrevious:4] // the tuple will have 5 values
skip:4] // skip until we have at least 5 recent values, no nils
map:^id(RACTuple *tuple) {
// calculate average
return [tuple.allObjects valueForKeyPath:@"@avg.self"];
}];
```