daf bit: Sukkah 38

Hallel, a collection of psalms (mostly), is recited every day during festivals, among other times. (If you don't go to services at these times, you may still know Hallel from the Pesach seder.) The g'mara on today's daf lays out the congregational responses, the custom being for the leader to recite and for the congregation to respond at certain places (usually but not always with "halleluyah"). So, what is the halacha of one who hears Hallel but doesn't say the responses? The g'mara says that he has fulfilled his obligation. How do we know this? From "even all the words of the book which the king of Judah had read" (II Kings 22:16) -- was it not Shafan who read them before the king (ibid 10)? Therefore he who listens, it's as if he read. But perhaps Josiah read it himself later? No, because it is written "because your heart was tender, and you did humble yourself before the Lord, when you heard what I spoke" (ibid 19) -- heard, not read. (38b)