| http://www.w3.org/ns/prov#value | - Thus, our preferred second compounds are those having the structure II or III where Y is an oxygen atom, a sulphur atom or a group having the structure ##STR5## as hereinbefore defined, or substituted derivatives of structures II and III having halogen atoms, monovalent hydrocarbon, ether or thioether groups or halogen substituted derivatives thereof, preferably halogen atoms, alkyl groups contain
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