The Gemara explains why we don’t add another bracha to Havineinu on motzei Shabbos (for havdala) since Havineinu was established specifically as a ‘seven bracha unit’ and not eight. If Havineinu assumes a new identity as a ‘seven bracha tefilla’ then why doesn’t the Gemara require a source corresponding to this number as the Gemara sought regarding the various types of shemone esrei (i.e. 18 of weekdays, 7 of Shabbos etc.)?
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