@Override
            public void visit(TraceNode node) {
                if (node.payload != null) {
                    out.append(node.payload).append(" ");
                }
            }
<p>Visits this <code>TraceNode</code> and all of its descendants in depth-first, prefix order.</p>

@param visitor The visitor to accept.
@return The <code>visitor</code> parameter.
vespajlib/src/main/java/com/yahoo/yolean/trace/TraceNode.java