private static Sorting.UcaSorter.Strength config2Strength(AttributesConfig.Attribute.Sortstrength.Enum s) {
        if (s == AttributesConfig.Attribute.Sortstrength.PRIMARY) {
            return Sorting.UcaSorter.Strength.PRIMARY;
        } else if (s == AttributesConfig.Attribute.Sortstrength.SECONDARY) {
            return Sorting.UcaSorter.Strength.SECONDARY;
        } else if (s == AttributesConfig.Attribute.Sortstrength.TERTIARY) {
            return Sorting.UcaSorter.Strength.TERTIARY;
        } else if (s == AttributesConfig.Attribute.Sortstrength.QUATERNARY) {
            return Sorting.UcaSorter.Strength.QUATERNARY;
        } else if (s == AttributesConfig.Attribute.Sortstrength.IDENTICAL) {
            return Sorting.UcaSorter.Strength.IDENTICAL;
        }
        return Sorting.UcaSorter.Strength.PRIMARY;
    }
Check sorting specification makes sense to the search cluster before
passing it on to the backend.

@author Steinar Knutsen
container-search/src/main/java/com/yahoo/prelude/searcher/ValidateSortingSearcher.java