@Override
    public RangeBucketPreDefFunctionNode clone() {
        RangeBucketPreDefFunctionNode obj = (RangeBucketPreDefFunctionNode)super.clone();
        if (predef != null) {
            obj.predef = (ResultNodeVector)predef.clone();
        }
        return obj;
    }
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