Method unkeep

chapter09/src/main/java/com/seaofnodes/simple/node/Node.java:286–286

()

    public <N extends Node> N keep() { addUse(null); return (N)this; }
    // Remove bogus null.
    public <N extends Node> N unkeep() { delUse(null); return (N)this; }


    // Replace self with nnn in the graph, making 'this' go dead