theorem Th50:
  Sphere(|[a,b]|,r) = circle(a,b,r)
proof
  reconsider e = |[a,b]| as Point of Euclid 2 by TOPREAL3:8;
  thus Sphere(|[a,b]|,r) = Sphere(e,r) by Th13
    .= circle(a,b,r) by Th47;
end;
