public synchronized void setPhraseMatcher(PhraseMatcher matcher) {
        if (matcher == null)
            this.phraseMatcher = nullPhraseMatcher;
        else
            this.phraseMatcher = matcher;
    }
Thread safely sets the phrase matcher to use in this, or null to not use a phrase matcher
container-search/src/main/java/com/yahoo/prelude/semantics/RuleBase.java