Reference for Nedarim 31:8

אביי לא אמר טעם כרב אשי דלא קתני שאי אוכל

<b><i>GEMARA</i></b>. This proves that 'Oh oath that I eat of yours implies that I will not eat. Now this contradicts the following: Oaths are of two categories, which are extended to four, viz., '[I swear] that I will eat,' 'that I will not eat,' 'that I have eaten, 'that I have not eaten'.<span class="x" onmousemove="('comment',' The two categories are affirmative and negative oaths referring to the future, which are extended to include similar oaths in the past. ');">⁶</span>

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