Sentence:  In the Analytical phase of their Cubism , Braque and Picasso had not only had to minimize three-dimensionality simply in order to <WSD>preserve</WSD> it ; they had also had to generalize it - to the point , finally , where the illusion of depth and relief became abstracted from specific three-dimensional entities and was rendered largely as the illusion of depth and relief as such : as a disembodied attribute and expropriated property detached from everything not itself . 
Meaning of the word marked with <WSD> tags:  to keep up and reserve for personal or special use