| http://www.w3.org/ns/prov#value | - One structural variation shown in FIG. 5, as compared with the structures shown in FIGS. 1-4, is the fact that the base portion 11a, while having a curved lower rocking surface 14a, is open at its front and rear sides, and a plate 60a of heavy metal, such as iron, or other heavy density material, is inserted at a vertical altitude, between the underside of shelf 16a and the upper surface of the ar
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