public function lock( $args, $assoc_args ) {
		if ( ! Book::isBook() ) {
			WP_CLI::warning( 'Not a book. Did you forget the --url parameter?' );
			return;
		}
		if ( CustomCss::isCustomCss() ) {
			WP_CLI::warning( "Deprecated! Can't lock a theme if it's Custom CSS" );
			return;
		}

		$lock = new Lock();
		if ( $lock->isLocked() ) {
			WP_CLI::warning( 'Theme already locked.' );
		} else {
			$data = $lock->lockTheme();
			if ( $data === false ) {
				WP_CLI::error( 'Theme could not be locked.' );
			} else {
				$this->updateThemeLockOption( true );
				WP_CLI::success( 'Theme was locked: ' . wp_json_encode( $data ) );
			}
		}
	}