protected function pbCheck( $copyright_file ) {
		$result = $this->getZipContent( $copyright_file );

		foreach ( $result->body->div->div->p as $node ) {

			if ( strpos( $node->a['href'][0], 'pressbooks.com', 0 ) ) {
				$this->isPbEpub = true;
			}
		}
		// applies to PB generated EPUBs with PB_SECRET_SAUCE
		// @see createCopyright() in export/epub/class-pb-epub201.php
		if ( 'copyright-page' === $result->body->div[0]->attributes()->id[0] && 'ugc' === $result->body->div->div->attributes()->class[0] ) {
			$this->isPbEpub = true;
		}
	}