act on the rail to urge it against the wall of the first arm 8 10 of the passages 4 6 in this case the tongue is preferably defined with a flexibility range able to take up maximum variations to ensure that it remains permanently under pressure against the rail the tongues could also be offset in the direction of translation of the carrier member or a greater number of tongues could be provided three tongues can for example be provided with one at one side of the rail and two tongues at the other side of the rail at both sides of the first tongue in the longitudinal direction of the rail this 3 tongue configuration ensures a guiding effect without an accompanying increase in the surface of contact between the tongues and the rail four tongues can be provided in the form for example of two sets of two tongues facing each other this solution like the 3 tongue solution improves guiding it is also clear that the invention is not limited to the preferred embodiment for the rail the third rail portion and second arm 14 16 of passages 4 6 are designed to ensure the carrier member is retained in the y direction they could have a different shape more generally the tongues could apply a force to the rail in the x direction but via a portion other than the one ensuring guiding in this direction for example by bearing against the end of the third rail portion the rail can have a different shape u2014 with for.