@Override
    public String toString() {
        StringBuilder ret = new StringBuilder();
        for (ReferenceNode node : this) {
            ret.append(node).append(" ");
        }
        return ret.toString();
    }
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