(Ibid.) "after she had been defiled": This tells me only (that this is so) from marriage (with the first) to marriage (with the second). Whence do I derive (the same for) from betrothal to marriage, from marriage to betrothal, from betrothal to betrothal? From "Her first husband shall not be able to take her; "her first to take her," "who had sent her away to take her," "to return to take her."
R. Yossi b. Kipper says in the name of R. Elazar b. Azaryah: From betrothal (to the second) she is permitted (to return to the first); from marriage she is forbidden, it being written "after she has been defiled" (i.e., cohabited with). And the sages say: Both from betrothal and marriage she is forbidden. Why, then, is it written "after she has been defiled"? To include (as forbidden to return to him) a sotah (a woman suspected of adultery) who had secreted herself (with the one she was warned against.)