| http://www.w3.org/ns/prov#value | - R1 to R3 are as hereinbefore defined, and Z1 denotes a leaving group such as a halogen atom, a substituted hydroxy, mercapto, sulphinyl, sulphonyl or sulphonyloxy group such as for example a chlorine or bromine atom, a methanesulphonyl or methanesulphonyloxy group,
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