theorem
  for a,b,c being POINT of G st a on b*c holds b=a or b=c or c on b*a
proof
  let a,b,c be POINT of G;
  assume that
A1: a on b*c & b<>a and
A2: b<>c;
  b*c = c*b by A2,Th16;
  hence thesis by A1,A2,Th35;
end;
