@Override
        public void printNode(final StringBuilder sb) {
            final OperatorNode operatorNode = this;
            sb.append(" ");
            sb.append(operatorNode.getFilterOperationMode().toString());
            sb.append(" ");
            sb.append("(");

            // print each of the child nodes
            for (final PrintableNode childNode : operatorNode.getChildren()) {
                childNode.printNode(sb);
                sb.append(",");
            }
            // remove the trailing comma
            if (sb.charAt(sb.length() - 1) == ',') {
                sb.deleteCharAt(sb.length() - 1);
            }

            sb.append(")");
        }
Need to supply a list of children as there is no point in creating an
operator node without children

@param childNodes
stroom-statistics/stroom-statistics-impl-sql/src/main/java/stroom/statistics/impl/sql/search/FilterTermsTree.java