Apparatus 1 wherein said transverse inverted V shaped fingers have included surfaces which lie in a pair of intersecting planes W and X, wherein said V shaped groove has included surfaces which lie in {HEAD ~ a pair} of intersecting planes Y and Z, wherein plane W intersects with plane Y in an interior angle γ, wherein plane X intersects with plane Z in {TAIL ~ an interior angle} δ, wherein γ>90°, and wherein δ>90° 
Apparatus 1 wherein said transverse inverted V shaped fingers have included surfaces which lie in a pair of intersecting planes W and X , wherein said V shaped groove has included surfaces which lie in { HEAD ~ a pair } of intersecting planes Y and Z , wherein plane W intersects with plane Y in an interior angle γ , wherein plane X intersects with plane Z in { TAIL ~ an interior angle } δ , wherein γ>90 ° , and wherein δ>90 °
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