private static String getFeatureSetKey(Predicate predicate) {
        if (predicate instanceof FeatureSet) {
            return ((FeatureSet) predicate).getKey();
        } else {
            Negation negation = (Negation) predicate;
            return ((FeatureSet) negation.getOperand()).getKey();
        }
    }
A FeatureConjunction is a special type of Conjunction where
all children are either a FeatureSet or a Negation (with an underlying FeatureSet).
The underlying FeatureSets may only have one value.

@author bjorncs
predicate-search-core/src/main/java/com/yahoo/document/predicate/FeatureConjunction.java