| http://www.w3.org/ns/prov#value | - According to one embodiment of the invention (see FIGS. 2-3), there is provided a substantially horizontal, detachable, preferably substantially conical, base which either temporarily completely (see FIG. 1), or continuously partially (see FIGS. 2 and 3), blocks the opening in the lower end of the outer wall of the BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view, partially in section, of a
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