public function invocation( $isbn, $isbn_text_font, $isbn_text_size, $text_font, $text_size ) {

		$ps[] = "50 50 moveto ({$isbn}) (includetext isbntextfont={$isbn_text_font} isbntextsize={$isbn_text_size} textfont={$text_font} textsize={$text_size})";
		$ps[] = '/isbn /uk.co.terryburton.bwipp findresource exec';

		return implode( "\n", $ps ) . "\n";
	}