public static boolean isValidFeatureConjunctionOperand(Predicate operand) {
        return operand instanceof Negation
                    && ((Negation) operand).getOperand() instanceof FeatureSet
                    && isValidFeatureConjunctionOperand(((Negation) operand).getOperand())
                || operand instanceof FeatureSet && ((FeatureSet) operand).getValues().size() == 1;
    }
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