function updateReferences (el) {
      const id = 'placemark_marker_' + el.id;
      const markerName = 'marker-start';
      const marker = getLinked(el, markerName);
      if (!marker || !marker.attributes.class) { return; } // not created by this extension
      const url = el.getAttribute(markerName);
      if (url) {
        const len = el.id.length;
        const linkid = url.substr(-len - 1, len);
        if (el.id !== linkid) {
          const val = $id('placemark_marker').getAttribute('value') || 'leftarrow';
          addMarker(id, val);
          svgCanvas.changeSelectedAttribute(markerName, 'url(#' + id + ')');
          svgCanvas.call('changed', selElems);
        }
      }
    }
Called when the main system creates or modifies an object.
Its primary purpose is to create new markers for cloned objects.
@param {Element} el
@returns {void}