The apparatus comprises means forming a plurality of mutually parallel blades extending outward at the first surface at an acute angle with {HEAD ~ respect} to {TAIL ~ a direction} normal to the first surface, and means forming a plurality of mutually parallel slots extending inwardly at the second surface at the acute angle with respect to a direction normal to the second surface 
The apparatus comprises means forming a plurality of mutually parallel blades extending outward at the first surface at an acute angle with { HEAD ~ respect } to { TAIL ~ a direction } normal to the first surface , and means forming a plurality of mutually parallel slots extending inwardly at the second surface at the acute angle with respect to a direction normal to the second surface
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