Returns a cap with the given center and surface area. Note that the area
can also be interpreted as the solid angle subtended by the cap (because
the sphere has unit radius). A negative area yields an empty cap; an
area of 4*Pi or more yields a full cap. "center" should be unit length.
Returns a cap with the given center and surface area. Note that the area can also be interpreted as the solid angle subtended by the cap (because the sphere has unit radius). A negative area yields an empty cap; an area of 4*Pi or more yields a full cap. "center" should be unit length.