@Override
    protected void appendTo(StringBuilder out) {
        for (Iterator<Predicate> it = operands.iterator(); it.hasNext(); ) {
            Predicate operand = it.next();
            if (operand instanceof Disjunction) {
                out.append('(');
                operand.appendTo(out);
                out.append(')');
            } else {
                operand.appendTo(out);
            }
            if (it.hasNext()) {
                out.append(" conj ");
            }
        }
    }
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