Niddah

Our Rabbis taught: A woman may attribute a stain  to another woman  who was awaiting a day for a day, if it  was the latter's second day,  and  to a woman  who counted seven days  before she had performed ritual immersion.  Hence she is at an advantage  while her friend is at a disadvantage;  so R. Simeon b. Gamaliel. Rabbi ruled, She  may not so attribute it.  Hence both are at a disadvantage. They  agree, however, that she may attribute a stain to a woman who was awaiting a day for a day if it  was the latter's first day,  and to a woman who was abiding in her clean blood,  and to a virgin whose blood is clean.  Why was it necessary to state the 'hence' of R. Simeon b. Gamaliel?  — On account of the ruling of Rabbi.  Why was it necessary to state the 'hence' of Rabbi?  — It might have been presumed that only the woman on whom the stain was found shall be at a disadvantage while the other shall not be disadvantaged, hence we were informed that both are at a disadvantage. R. Hisda stated: If a clean and an unclean person walked respectively in two paths one of which was clean and the other unclean,  we arrive at the dispute between Rabbi and R. Simeon b. Gamaliel.  R. Adda demurred: Rabbi may have maintained his view only  there, because both are in similar conditions,  but what difference [to the unclean person in this case] could our assumption make?  And R. Hisda?  — After all  she has yet to perform the immersion.  It was stated:  R. Jose son of R. Hanina ruled, If a clean and an unclean person, and even if a clean, and a doubtfully clean person walked respectively in two paths one of which was unclean and the other clean, it may be assumed, according to the opinion of all,  that the unclean path was taken by the doubtfully clean person and the clean path by the clean one. R. Johanan enquired of R. Judah b. Liwai: May a stain  be attributed to [another woman  who was unclean on account of] a stain? So far as Rabbi's view is concerned the question does not arise; for, since in that case  where the woman had observed a discharge from her own body  you said [that the other woman's stain] may not be attributed [to her], how much less then may this be done in this case where the stain may have originated from an external cause.  The question arises only in connection with the view of R. Simeon b. Gamaliel: Is it only in that case,  where the woman had observed a discharge from her own body,  that the other woman's stain may be attributed to her, but here, where the stain may have originated from an external cause,  she may not so attribute it,  or is it possible that no difference is made between the two cases? — The other replied: One may not so attribute it. What is the reason? — Because [there is a tradition that]  one may not so attribute it. He pointed out to him the following objection: 'Is it not permissible to attribute a stain  to [another woman  who was unclean on account of] a stain. If a woman  had lent her shirt to a gentile woman or to one who continued unclean by reason of a stain, she may attribute its to her.  (But is not this Baraitha self contradictory: In the first clause you stated, 'it is not permissible to attribute' while in the final clause you stated that it was permissible to attribute? — This is no difficulty: The former is the view of Rabbi while the latter is that of R. Simeon b. Gamaliel. There are some who read: The latter as well as the former represents the view of Rabbi, but  the latter  applies to her first day  while the former  applies to her second day.  R. Ashi replied: The former  as well as the latter  represents the view of R; Simeon b. Gamaliel and yet there is no difficulty,