incision 24 along the channel 28 the anchor tab 18 is thus fixed in the three directions of space the channel 28 has essentially the dimensions of the anchor tab 18 the width wd of the channel in the distal region is about two or three times the width wp of the channel in the proximal region due to the resiliency of the foam material 26 the size of the channel 28 can be a little smaller than the anchor tab 18 which results in a tight and durable fit of the anchor tab 18 to the seat foam 26 fig5 7 illustrate a second embodiment of the invention which differs from the first embodiment in that the incision comprises a first narrow vertical slit 36 and a second narrow vertical slit 38 the first slit 36 extends over the entire length of the channel 28 in the distal region of the channel the vertical slit 38 extends transversally with respect to the first slit 36 to insert the anchor tab 18 into the channel 28 the slits 36 38 are widened to let the anchor tab 18 pass as the slits 36 38 are so narrow that the lobes 30 32 34 a 34 b of the channel roof are in contact or almost in contact when the anchor tab 18 is lodged in the incision the latter is hardly perceptible by touch fig8 10 illustrate a third embodiment of the invention the incision is in this case a cavity 24 that extends substantially vertically with respect to the surface of the seat foam and whose vertical cross section exhibits generally the shape of an inverted t the cavity 24 has an.