| http://www.w3.org/ns/prov#value | - --Xr --Y4 -- where r is an integer at least 1, each X is independently C or O, and each C can independently be substituted or unsubstituted, saturated or unsaturated, and Y4 is selected from the group consisting of, O, S, NH, N???N and a direct bond linking Xr with R1, R2, or R3 ; and Each of Y1, Y2, and Y3 is independently selected from the group consisting of O and S. 21.
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