Game theory says you need to understand your "outside options": what the alternative to a deal with this group is. How well you do depends on how good you can make all the outside options. Here's a go at the decision tree Clegg faces.

It's a wikimap, so feel free to add your view of it or tease out more or different conclusions from mine.

The logic of my version is:

• A deal with Labour now is unlikely to deliver PR, which is what would make it worthwhile from LD's point of view
• A deal with Tories will not deliver PR
• A Tory minority government risks giving Tories a majority in 9 months and missing the historic moment
• Clegg will be very tempted to do a deal with the Tories with inbuilt failure so that LD's can return to a PR negotiation with other parties in 9 months
• Present that deal as stesman-like: "The economic crisis demands it, and I will not make the country suffer for PR ... but once the economy is on the mend, I will insist on my PR reward"