@Override
    protected void onDeserialize(Deserializer buf) {
        super.onDeserialize(buf);
        predef = (ResultNodeVector)deserializeOptional(buf);
    }
Obtain the predefined bucket list of this bucket expression

@return predefined bucket list for this expression
searchlib/src/main/java/com/yahoo/searchlib/expression/RangeBucketPreDefFunctionNode.java