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