private void addRankProperty(RankProperty rankProperty) {
        // Just the usual multimap semantics here
        rankProperties.computeIfAbsent(rankProperty.getName(), (String key) -> new ArrayList<>(1)).add(rankProperty);
    }
Returns a read only map view of the rank properties to use in this profile. This is never null.
config-model/src/main/java/com/yahoo/schema/RankProfile.java