[hughc]

This talk of contracts sounds to me just like Android's intents, which have been in there forever - one example being a app having a share button that then pops up a list of all apps that offer an endpoint for a particular data type (ie share this photo by email / sms / dropbox / google plus / facebook / any other app you may have installed) - the hosting app has no idea about the endpoints, it just casts around and offers them to the user.

It's in scenarios like this where the back button makes perfect sense, where you change context midstream, and then wish to return to the original one. I recall on an iPhone in a similar scenario (emailing a photo) that there was no obvious way back to the place the photo came from - once you switched the mail app, it felt like a UX dead-end. Change your mind about sending the mail and you're stuck. Back works intuitively by comparison. It can even work in chains- snap a photo, share via an app that does image resizing, share the result again via email; back -> back -> and you're back in the camera app.