Responsa for Shabbat 19:6
אֵלּוּ הֵן צִיצִין הַמְעַכְּבִין אֶת הַמִּילָה, בָּשָׂר הַחוֹפֶה אֶת רֹב הָעֲטָרָה. וְאֵינוֹ אוֹכֵל בַּתְּרוּמָה. וְאִם הָיָה בַעַל בָּשָׂר, מְתַקְּנוֹ מִפְּנֵי מַרְאִית הָעָיִן. מָל וְלֹא פָרַע אֶת הַמִּילָה, כְּאִלּוּ לֹא מָל:
These are the tzitzin [strands of flesh remaining from the foreskin] which (if they remain) invalidate milah: Flesh which covers most of the corona. And he (one with such tzitzin) does not eat terumah [if he were a Cohein. For an uncircumcised Cohein is forbidden to eat terumah, it being written in respect to the Pesach offering (Exodus 12:45): "A sojourner and a hired man may not eat of it," and, in respect to terumah, (Leviticus 22:10): "The sojourner with a Cohein and his hired man may not eat the holy thing" — Just as the Pesach offering is forbidden to one who is uncircumcised, so, terumah.] And if he were fat, [so that after the entire foreskin had been removed, the organ still seems to be covered by flesh], it is corrected [i.e., the thickness is reduced with the knife] because of "appearances" [i.e., so that he not give the appearance of being uncircumcised.] If he were circumcised, but the milah were not exposed (by pulling down the membrane), it is as if he had not been circumcised, [and he must go back and expose it, even if he had withdrawn. And as long as he is engaged in circumcision on Shabbath, he cuts both those tzitzin that invalidate milah and those which do not. After he withdraws, he returns for invalidating tzitzin but not for non-invalidating tzitzin.]