public function _addFont(PHPExcel_Style_Font $font)
	{
		$fontHashCode = $font->getHashCode();
		if(isset($this->_addedFonts[$fontHashCode])){
			$fontIndex = $this->_addedFonts[$fontHashCode];
		} else {
			$countFonts = count($this->_fontWriters);
			$fontIndex = ($countFonts < 4) ? $countFonts : $countFonts + 1;

			$fontWriter = new PHPExcel_Writer_Excel5_Font($font);
			$fontWriter->setColorIndex($this->_addColor($font->getColor()->getRGB()));
			$this->_fontWriters[] = $fontWriter;

			$this->_addedFonts[$fontHashCode] = $fontIndex;
		}
		return $fontIndex;
	}