code: function getRefsTo(elem, iterator) {
          var id,
              ref,
              obj,
              list = [];
          if (elem) {
              obj = _refMap[elem._id];
              if (obj) {
                  for (id in obj) {
                      if (obj.hasOwnProperty(id)) {
                          ref = _idMap[id];
                          if (iterator) {
                              if (iterator(ref)) { list.push(ref); }
                          } else {
                              list.push(ref);
                          }
                      }
                  }
              }
          }
          return list;
      }
docs: Return all elements referencing to the given element.
  @param {Core.Element} elem Element. (model element, view element, or diagram)
  @param {?function(Element):boolean} iterator if given, returns instances only satisfying iterator function.
  @return {Array<Core.Element>} Elements referencing to.