Praying on a Rug That Counts the Rak‘ahs

Al Liwaa:  What is the ruling on praying on a rug that counts the prayer’s rak‘ahs during the prayer, especially for the elderly and others who often become forgetful? — The principle of alerting someone to an error in prayer is prescribed; a follower is instructed to say “Subhan Allah” if the imam makes a mistake if the follower is a man, and to clap if the follower is a woman. Indeed, it is also permissible to alert someone who is outside…