code: public double getUniqueKeywordValue(String keyword) {
          Collection<Double> values = getAllKeywordValues(keyword);
          if (values != null && values.size() == 1) {
              return values.iterator().next();
          }
          return NO_UNIQUE_VALUE;
      }
docs: Returns the unique value that this keyword matches, or {@link #NO_UNIQUE_VALUE}
  if the keyword matches multiple values or is not defined for this PluralRules.
  
  @param keyword the keyword to check for a unique value
  @return The unique value for the keyword, or NO_UNIQUE_VALUE.