public Optional<int[]> getConjunctionIdsForFeature(long featureId) {
            return Optional.ofNullable(map.get(featureId));
        }
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