predicate/predicate.md: Repeat warning that portions of this document are not yet implemented.
predicate: Fix several issues with singular ValueLists.  (see below) (RNS-774)

With the move to graph evaluation state (eval.*), ValueLists of unfinished nodes may be singular.  This can happen, for example, for a node that may eventually alias a child, but may also setup a local valuelist and hasn’t made that decision yet.