[], literal(true) => head in Nodes;
[forall node in Nodes], !(node.next=literal(0)) => node.next in Nodes;
[forall node in Nodes], !(node.next=literal(0)) => <node, node.next> in nextnodes;
[forall node in Nodes], !(node.prev=literal(0)) => <node, node.prev> in prevnodes;