desc  - description of the key's context
xpath - a function returning the XPath expression
        to retrieve this context's collection
func  - function bound to the key
value - just a value that key returns
        (currently only used for up|down)
coll  - list of items to navigate (refreshed on context change)
index - cursor (index into coll)
focus - set focus on the element found by evaluating 'xpath' 
<c>   - sub-hash (subcontext) entry (<c> is a single char)