@Override
    public List<Predicate> getOperands() {
        return operands;
    }
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