ed on which items it contains. The function should return the
                    properties to
                    create the cluster node. In the example below, we ensure preservation of mass and value when forming
                    the
                    cluster:
<pre class="prettyprint lang-js">
var options = {
  processProperties: function (clusterOptions,
      childNodes, childEdges) {
    var totalMass = 0;
    var totalValue = 0;
    for (var i = 0; i < childNodes.length; i++) {
      totalMass 