Midrash for Sanhedrin 37:2
גזירה ביאה ראשונה אטו ביאה שניה
The positive command<span class="x" onmousemove="('comment',' Of yibbum. — This is a general rule, where two precepts come into opposition. ');"><sup>3</sup></span> should set aside the negative? — The first act of connubial intercourse was forbidden as a preventive measure against further acts.<span class="x" onmousemove="('comment',' Which would be a transgression, the precept having been fulfilled by the first. ');"><sup>4</sup></span>
Sifrei Devarim
"who have the contention": Let the owner of the (disputed) ox come and stand by his ox.
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