@Override
    public int hashCode() {
        int ret = 0;
        for (ReferenceNode node : features) {
            ret += node.hashCode() * 17;
        }
        return ret;
    }
Returns the feature at the given index.

@param i the index of the feature to return.
@return the feature at the given index.
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/FeatureList.java