code: static public function isBetterState($stateA, $stateB)
      {
          // TODO Find something more explicit than 'better' (availability ?)
  
          // TODO assert valid states ?
  
          if ($stateA === static::STATE_IN_STOCK) {
              return $stateB !== static::STATE_IN_STOCK;
          } elseif ($stateA === static::STATE_PRE_ORDER) {
              return $stateB === static::STATE_OUT_OF_STOCK;
          }
  
          return false;
      }
docs: Returns whether the state A is better than the state B.
  
  @param string $stateA
  @param string $stateB
  
  @return bool