| http://www.w3.org/ns/prov#value | - tter containing at least one of said R.sub.1 radicals and a metal or metalloid comprising Al, Ge, Pb, P, Li, Sn, Sb, Te, Se, As or B, with the proviso that such organometal or metalloid group may itself comprise one or more--O.sup.16 --O.sup.17 groups; or one of the radicals: ##STR5## in which R.sub.1 is as above defined and M is a hydrogen atom or an alkali metal, said radicals R.sub.1 containing
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