code: private function setCache($singular, $plural = null)
  	{
  		$singular = StringHelper::strtolower($singular);
  
  		if ($plural === null)
  		{
  			$plural = $singular;
  		}
  		else
  		{
  			$plural = StringHelper::strtolower($plural);
  		}
  
  		$this->cache[$singular] = $plural;
  	}
docs: Sets an inflected word in the cache.
  
  @param   string  $singular  The singular form of the word.
  @param   string  $plural    The plural form of the word. If omitted, it is assumed the singular and plural are identical.
  
  @return  void
  
  @since   1.0