code: function extractChanged(operation) {
          var i, len, op, elem, changed = [];
          if (operation.ops.length > 0) {
              for (i = 0, len = operation.ops.length; i < len; i++) {
                  op = operation.ops[i];
                  if (op._elem && op._elem._id) {
                      elem = get(op._elem._id);
                      if (elem && !_.contains(changed, elem)) {
                          changed.push(elem);
                      }
                  }
                  if (op.arg && op.arg._id) {
                      elem = get(op.arg._id);
                      if (elem && !_.contains(changed, elem)) {
                          changed.push(elem);
                      }
                  }
              }
          }
          return changed;
      }
docs: Extract changed elements from a given Operation
  @param {Operation} operation
  @return {Array.<Element>}