| 摘要 |
<p>It comprises a level board (1) on which is carried out a hollow (2) with inside walls (3) perpendicular to one another with a groove (5) and an inside profile (7) in which comes to longitudinally slide an outside profile (9) of an other level board equal and complementary to the inside profile (7). The outside profile (9) consists of a beating parts (10) with surfaces (11, 12) perpendicular each other and protruding part (13) having the same shape and dimension of the groove (5).
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