| http://www.w3.org/ns/prov#value | - As said other components, the following may be exemplified in addition to the above-mentioned Li.sub.2 O, Na.sub.2 O, K.sub.2 O, SnO.sub.2, TiO.sub.2 and ZrO.sub.2. Namely, e.g. Bi.sub.2 O.sub.3, P.sub.2 O.sub.5 or F may be contained in order to
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