What is the ruling if the follower misses an entire rak'ah or more due to an excuse, such as continuous drowsiness?

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If a person sleeps through two rak'ahs behind the Imam, he makes up those two rak'ahs after the Imam concludes the prayer, as if he had caught those two rak'ahs. If the precedence is more than one pillar but less than a full rak'ah, and the excuse is then removed, the explicit narration from Ahmad is that the follower follows the Imam and that specific rak'ah is not counted. This is exemplified by the case where the Imam bows and prostrates while the follower is still standing, unaware, in which case the follower should prostrate with the Imam and then perform an extra rak'ah later.