Salinon
this is a special shape called a salinon ( the figure defined by curves q,r,s, and t) it has an area which is alway equal to that of a circle which is defined by the curves p and k.
as in any good mathematical truth one can see that the boundary condition (where point B sits on point E) constitutes where the two area are congruent and so this is simply a special case of a circle!