Reference for Makkot 16:10

שוליא דנגרי חיותא היא דלמדיה דגמיר אומנותא אחריתי:

Learning [to be] a carpenter's apprentice is a livelihood that he is teaching him! [Actually] he is teaching him another trade [that is not his livelihood—so striking and killing him inadvertently will result in exile).

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