Peter's friends

One thing in the discussions last night we want a want of referring to "Peter's friends" as well as "Peter".

"Peter" is a resource, and so is "Peter's friends". 

"Peter's friends" can be paginated when it becomes too large.

"Peter's friends" can be appended to. 
e.g. 
simply POSTing "Mary" to "Peter's friends" will assert this new friendship. 

We called "Peter's friends" a value set, and it is a useful construct it seems. 

What this could mean, is that a set of such atomic operations, i.e. "Add Mary to  Peter's friends", "Add Andrew to Peter's friends", "Remove Susan from Peter's friends", ... is a PATCH format (?) 

Roger