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