| http://www.w3.org/ns/prov#value | - Recording materials which are of particular interest are those wherein the compound of the formula V is a compound of the formula Va ##STR17## in which R13 is hydrogen, C1 -C4 alkyl or a group ##STR18## and R14, R15, R16, R17, R18, R'16, R'17 and R'18 are as defined above.
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