public RankingExpressionFunction getFirstPhase() {
        if (firstPhaseRanking != null) return firstPhaseRanking;
        return uniquelyInherited(RankProfile::getFirstPhase, "first-phase expression").orElse(null);
    }
Returns the ranking expression to use by this. This expression must not be edited.
Returns null if no expression is set.
config-model/src/main/java/com/yahoo/schema/RankProfile.java