Method unkeep

chapter21/src/main/java/com/seaofnodes/simple/node/Node.java:287–288

()

    public <N extends Node> N keep() { addUse(null); return (N)this; }
    // Remove bogus null.
    @SuppressWarnings("unchecked")
    public <N extends Node> N unkeep() { delUse(null); return (N)this; }
    // Test "keep" status
    public boolean iskeep() { return _outputs.find(null) != -1; }
    public void unkill() {