| http://www.w3.org/ns/prov#value | - A process according to claim 1 wherein the marking material is a dye selected from the group consisting of 5,10,15,20-tetrakis-(1-methyl-4-pyridyl)-21H,23H-porphine tetra-p-tosylate salt; 5,10,15,20-tetrakis-(1-methyl-4-pyridyl)-21H,23H-porphine tetrachloride salt; 5,10,15,20-tetrakis-(1-methyl-4-pyridyl)-21H,23H-porphine tetrabromide salt; 5,10,15,20-tetrakis-(1-methyl-4-pyridyl)-21H,23H-porphine
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