The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
65. (The meditations on members of sacrificial actions are) rather not (to be combined), as the text does not state their going together.
The words 'rather not' discard the pûrvapaksha. The meditations resting on members of actions are not to be treated like what they rest on, because scripture does not state their going together. Scripture actually states the going together of the Stotras and other subordinate members of sacrificial action which are enjoined in the three Vedas; cp. passages such as 'After the taking of the graha or the raising of the kamasa he performs the Stotra;' 'After the Stotra he recites;' 'Prastotri sing the Sâman;' 'Hotri recite the Yâgyâ for this; 'and so on. But, on the other hand, there are no analogous texts expressly teaching the going together of the meditations.--But the going together of the meditations is established by those texts which intimate the successive performance of the different constituent members of a sacrifice!--By no means, we reply. The meditations subserve the end of man, while the texts referred to by you establish only the going together of the udgîtha and the like which subserve the purpose of the sacrifice. That the meditations on the udgîtha and so on--although resting on
members of sacrificial acts--yet subserve the end of man only in the same way as the godohana vessel does, we have already explained under III, 3, 42.--And this very difference between members of sacrificial action and the meditations resting on them, viz. that the former subserve the purpose of the sacrifice while the latter subserve the end of man, is founded on the express teaching of scripture 1.--And the further two indicatory marks (pointed out by the pûrvapakshin in Sûtras 63 and 64) supply no reason for the going together of the meditations, because no direct scriptural statement may be constructed from them. Nor 2 does the fact that in each sacrificial performance all foundations of meditations are comprised, enable us to conclude that the meditations founded on them are to be combined also; for the meditations are not caused by what they rest on. The meditations, as resting on their foundations, would, it may be admitted, not exist if those foundations did not exist. But therefrom it does not follow that the going together of the foundations implies a necessary going together of the meditations; for as to this we have no direct scriptural statement.--From all this it results that the meditations may be performed according to one's liking.
284:1 A remark refuting the averment made in Sûtra 62.
284:2 And this is meant to refute the second interpretation given of Sûtra 64.