code: function update(v, i, params, {oldValue}) {
        switch (i) {
        case 'children': _unobserveChildren(oldValue); _observeChildren(v); break;
        case 'attrs':    _unobserveAttrs   (oldValue); _observeAttrs   (v); break;
        }
      }
docs: When parameters change, tear down and resetup observers.