private boolean validStrength(String strength) {
        return (strength.equals("PRIMARY") ||
                strength.equals("SECONDARY") ||
                strength.equals("TERTIARY") ||
                strength.equals("QUATERNARY") ||
                strength.equals("IDENTICAL"));
    }
Constructs a new instance of this class.

@param exp     The expression to evaluate.
@param locale  The locale to used for sorting.
@param strength The strength level to use.
container-search/src/main/java/com/yahoo/search/grouping/request/UcaFunction.java