public RankingExpression getFirstPhaseRanking() {
        RankingExpressionFunction function = getFirstPhase();
        if (function == null) return null;
        return function.function.getBody();
    }
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