@Override
    public ZCurveFunctionNode clone() {
        ZCurveFunctionNode obj = (ZCurveFunctionNode)super.clone();
        obj.dim = dim;
        return obj;
    }
Obtain the predefined bucket list of this bucket expression

@return what part of the time you have requested
searchlib/src/main/java/com/yahoo/searchlib/expression/ZCurveFunctionNode.java