Although the width of the grooves 3 may vary to a considerable extent, a width of 4 to 7 mm, preferably 6 mm, has proved avanta-15 tageous, while the surfaces 4 may be slightly narrower and have for example, a width of 3 to 6 mm, preferably 4 mm. De préférence, les rainures 3 ont une profondeur de 0,5 à 2 mm. Preferably, the grooves 3 have a ...