- unary link, e.g service(x)
- binary relationship, e.g. edit-media(u1,x)
- pattern match on representation of x, e.g. r-match( xpathExpr , x)
- pattern match on occurrance of x in representation, e.g match( xpathExpr , x)
Logical sentences build from such predicates can be evaluated against the current state of the client to either test if a given URI matches the denoted context or to find all URIs known to the client that match the predicate conjunction.
The next step will be to express the operations that are currently stated in the form of prose in hypermedia specifications in terms of the above predicate conjunctions and HTTP operations. For example could AtomPub's 'create entry' operation be expressed as the combination of the predicate conjunction match( '/service/workspace/collection@href',x) and POST. Where the match predicate evaluates to true for any URI x that is found as the value of the XPath expression.
This needs more precision of course, but it's a start.