The lyrics of the modern Japanese national anthem, Kimi ga Yo, were written in the Heian period, as was The Tale of Genji by Murasaki Shikibu, one of the first novels ever written. Murasaki Shikibu's contemporary and rival Sei Shōnagon's revealing observations and musings as an attendant in the Empress' court were recorded collectively as The Pillow Book in the 990s, which revealed the quotidian capital lifestyle. The Heian period produced a flowering of poetry including works of Ariwara no Narihira, Ono no Komachi, Izumi Shikibu, Murasaki Shikibu, Saigyō and Fujiwara no Teika. The famous Japanese poem known as the Iroha (いろは), of uncertain authorship, was also written during the Heian period.

Answer this question, if possible (if impossible, reply "unanswerable"): What famous poem with an unknown author was written during the Heian period?
Iroha