@Override
    public StandardDeviationAggregator copy() {
        return new StandardDeviationAggregator(getLabel(), getLevelOrNull(), getExpression().copy());
    }
Constructs a new instance of this class.

@param expression the expression to aggregate on.
container-search/src/main/java/com/yahoo/search/grouping/request/StandardDeviationAggregator.java