Reference for Yevamot 109:14

כתב קרא אחרינא (ויקרא כ, כא) ערות אחיו גלה

is] a negative commandment that is derived from a positive one<span class="x" onmousemove="('comment',' Her husband's brother shall go in unto her if she has no children. ');">²⁹</span> it has only the force of a positive commandment!<span class="x" onmousemove="('comment',' The penalty for the transgression of which is not that of kareth! ');">³⁰</span> — For this reason Scripture wrote another text,<span class="x" onmousemove="('comment',' Which, in view of the texts from Lev. XVIII, 16a and b, is superfluous. ');">³¹</span>

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