Ripple allows us to encode how much we trust each person. For example, I can trust A with $10K and B with $10K, and will then accept IOUs of up to $10K from each as valid dollars.
In the real world, this does not mean I would trust them both with $10K at the same time. Perhaps the risk associated with A and B is highly correlated, or perhaps I just don't want to risk $20K of my money with others. For example, I could trust A and B together with $15K. So if I already have a $10K IOU from A, I will only accept up to $5K IOU from B.
In general, I can see myself trusting 10 different people with $10K, but certainly not trusting them all together with $100K.
Does Ripple allow encoding correlated trust relationships, or at least a specification of "total" trust? For example, I could define some trust relationships, including $10K to A; and I will further define that my total trust is $50K. If I hold $20K IOUs from others, I will automatically only accept up to $6K from A. ($10K * (1 - $20K / $50K))