{HEAD ~ a convex edge}, wherein {TAIL ~ a trough} of the concave edge is aligned with an apex of the convex edge, 
wherein
{ HEAD ~ a convex edge } , wherein { TAIL ~ a trough } of the concave edge is aligned with an apex of the convex edge ,
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH