| 摘要 |
In a flat square slab with a smooth surface and tongue-like attachments perpendicular to the side surfaces, in order to provide a large number of layout combinations, each side surface (26, 27) has two projections (22, 22', 22"), wherein each projection (22) is formed on a corner edge (23) of two adjacent side surfaces (26) and one projection (22') is formed on the same side surfaces (26) at the other corner edge (24) and the projections (22") are arranged at a slightly greater distance from the edge corners (24, 25) on the other two side surfaces (27) than the length of the attachments (22, 22') looked at from the smooth surface of the slab.
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