@Override
    public String toString() {
        return "BooleanIndexDefinition [arity=" + arity + ", lowerBound="
                + lowerBound + ", upperBound=" + upperBound + ", densePostingListThreshold="
                + densePostingListThreshold + "]";
    }
Encapsulates values required for predicate fields.

@author lesters
config-model/src/main/java/com/yahoo/schema/document/BooleanIndexDefinition.java