void addMatchReferences(Set<String> matchReferences) {
        for (Iterator<Production> i = productions.iterator(); i.hasNext(); ) {
            Production term = i.next();
            term.addMatchReferences(matchReferences);
        }
    }
Returns an unmodifiable view of the productions in this
container-search/src/main/java/com/yahoo/prelude/semantics/rule/ProductionList.java