| http://www.w3.org/ns/prov#value | - As a development of the first preferred embodiment obtained by expanding the above concept, the optical depth of each groove 26 may be set to (2n+1)??/8 (n is a positive integer), the optical depth of each phase pit 38 formed on the land header portion 34 may be set to (2n???1???4m)??/8 (m is an integer not less than 0), and the optical depth of each phase pit 36 formed on the groove header portio
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