code: function(arg) {
      readOnly(this, ID, COUNT++);
      readOnly(this, "_listeners", new Listeners());
      readOnly(this, "_links", []);
      // An object  with the  attribute `isContentChangeAware  === true`
      // can fire a property value changed when its contant has change.
      readOnly(this, "isContentChangeAware", true);
      if (Array.isArray(arg)) {
          readOnly(this, "_array", arg);
      } else if (isList(arg)) {
          readOnly(this, "_array", arg._array);
          this.link(arg);
      } else {
          readOnly(this, "_array", [arg]);
      }
  }
docs: A list is an observable array.
  
  Several lists can share the same internal array. In that case, when
  one list modifies the array, all  lists will emit the same event as
  it. Such lists are linked.