Method put

chapter24/src/test/java/com/seaofnodes/simple/Eval2.java:44–47

(Node n, Object d)

        // Put a mapping from Node->Object
        Node put(Node n, Object d) {
            assert d!=null;
            return put0(n,d);
        }
        // Put a mapping from Node->Object?
        Node put0(Node n, Object d) {
            assert _refcnt>0;