theorem
  s in circle(a,b,r) & t in circle(a,b,r) implies LSeg(s,t) /\ circle(a,
  b,r) = {s,t}
proof
  reconsider G = |[a,b]| as Point of Euclid 2 by TOPREAL3:8;
  Sphere(G,r) = circle(a,b,r) by Th47;
  then
A1: Sphere(|[a,b]|,r) = circle(a,b,r) by Th13;
  assume s in circle(a,b,r) & t in circle(a,b,r);
  hence thesis by A1,Th33;
end;
