| http://www.w3.org/ns/prov#value | - Preferably D4 is linked to D3 by means of a substantially horizontal surface which may be aligned with the lower surface of upper portion 18.Viewed in cross-section, as in FIGS. 1 and 2, the upper surface of upper portion 18 is preferably convex: from the inside outwards there is a first ascending section (conical in the figures) and a second descending section (convex in the figures), the outer p
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