Modelling trust lines between counter parties

I've been thinking lately about implementing a web version of the ripple
protocol [1]

One of the key concepts in ripple is adding of trust lines.

So I can say Alice trusts Bob for $10

This means that Bob now has $10 credit with Alice, and if you allow this to
be transitive you can set up a network of trust where people vouch for each
other and can allow a basic form of clearing on the web

I'm wondering about 2 things.

1. How should we model trust between two People?

This is similar to the existing Transfer and Credit classes in that you
have two counter parties, an amount and a currency.

2. Should it be added to the commerce vocabulary? [2]

[1] http://en.wikipedia.org/wiki/Ripple_(payment_protocol)
[2] https://web-payments.org/specs/source/vocabs/commerce.html