code: protected function drawBorderTop($isCore = false)
  	{
  		$crossroads = $isCore ? static::CROSSROADS_UP : static::CROSSROADS;
  
  		$this->drawBorder(static::BORDER_NW, $crossroads, static::BORDER_NE);
  		
  		$this->geometry->increaseNesting();
  	}
docs: Draw the top border.
  
  @author	Andrea Marco Sartori
  @param	boolean	$isCore
  @return	void