against ascent one and most preferably not more than one of the anchor slips 58 u2032 comprises unidirectional teeth 64 in one set and u201c dummy teeth u201d 97 as the other the latter are characterized by blunt rather than sharp edges 64 for instance by termination in rounded edges 98 when compared to the sharp teeth 64 of the rest of the anchor slips 58 in addition the u201c dummy teeth u201d 97 furthermore lack a preferred orientation of the teeth 97 rather they are symmetrical i e not slanted as fig1 b shows in contradistinction to the typical teeth 64 with a preferred orientation shown un fig1 a i estimate that the radius of the cylindrical curvature of the rounded edges 98 should not be less than about 0 4 mm preferably not less than about 0 8 mm to meet the object of the invention in other words the set of dummy teeth 97 opposes scant resistance in either axial direction against sliding along the casing wall this overcomes the potential problem of the teeth 64 u201c merging u201d or u201c integrating u201d with the casing after a long period of being together in the same biting position what happens is that as an anchor release operation begins the typical set of teeth 64 which partner the set of dummy the teeth 98 becomes unstuck freely and separates from the casing promoting immediate collapse.