code: public static function getConversionGroupUnits($group = null)
      {
          $conversionGroups = [];
          foreach (self::$conversionUnits as $conversionUnit => $conversionGroup) {
              if (($group === null) || ($conversionGroup['Group'] == $group)) {
                  $conversionGroups[$conversionGroup['Group']][] = $conversionUnit;
              }
          }
  
          return $conversionGroups;
      }
docs: getConversionGroupUnits
  Returns an array of units of measure, for a specified conversion group, or for all groups.
  
  @param string $group The group whose units of measure you want to retrieve
  
  @return array