Shabbat

is an affirmative precept:  thus there is an affirmative and a negative precept in respect of Festivals, and an affirmative precept cannot supersede a negative and an affirmative precept. Thus it [the burning of defiled terumah] is forbidden only on Festivals, but on weekdays it is well.  What is the reason? Said Rab: Just as it is obligatory to burn defiled sacred food, so t is obligatory to burn defiled terumah, and the Torah said, When it is burnt, you may benefit therefrom. Where did the Torah say thus? — [It follows] from R. Nahman's [dictum]. For R. Nahman said in Rabbah b. Abbuha's name, Scripture saith, And I, behold, I have given thee the charge of mine heave-offerings:  the Writ refers to two terumoth,  viz., clean and unclean terumah, and the Divine Law said'[I have given] thee', [meaning], let it be thine for burning it under thy pot. Alternatively, [it follows] from R. Abbahu's [dictum]. For R. Abbahu said in R. Johanan's name: 'Neither have I put away thereof, being unclean:'  'thereof' you may not 'put away,'  but you may 'put away' [burn] defiled oil of terumah. Yet [perhaps] say: 'thereof' you may not 'put away', but you may 'put away undefiled oil of kodesh  which is defiled? — Does it [the reverse] not follow a fortiori: if tithe, which is light,  yet the Torah said, neither have I put away thereof, being unclean'; then how much more so kodesh, which is more stringent? If so, in the case of terumah too let us say, does it [the reverse] not follow a afortiori?  — Surely thereof' is written!  And why do you prefer it thus?  — It is logical that I do not exclude kodesh, since it is [stringent] in respect of (Mnemonic: Pa NaK'aKaS):  [i] Piggul, [ii] Nothar, [iii] sacrifice [Korban], [iv] Me'ilah, [v] Kareth, and [vi] 'it is forbidden [asur] to an onen.  On the contrary, terumah is not to be excluded, since [it is stringent] in respect of its (mnemonic Ma HPaZ): [i] Death [Mithah], [ii] a fifth [Homesh],