public void writeToOutputStream(DataOutputStream out) throws IOException {
        SerializationHelper.writeIntArray(zList, out);
        SerializationHelper.writeLongArray(idMapping, out);
        out.writeInt(kIndex.size());
        for (IntObjectPair<FeatureIndex> p : kIndex.keyValuesView()) {
            out.writeInt(p.getOne());
            p.getTwo().writeToOutputStream(out);
        }
    }
A searchable index of conjunctions (see {@link FeatureConjunction} / {@link IndexableFeatureConjunction}).
Implements the algorithm described in the paper
<a href="http://dl.acm.org/citation.cfm?id=1687633">Indexing Boolean Expressions</a>.

@author Magnar Nedland
@author bjorncs
predicate-search/src/main/java/com/yahoo/search/predicate/index/conjunction/ConjunctionIndex.java