Method keep

chapter14/src/main/java/com/seaofnodes/simple/node/Node.java:256–256

()

    // Shortcuts to stop DCE mid-parse
    // Add bogus null use to keep node alive
    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; }
    // Test "keep" status