theorem Th28:
  for a,b,c,d being POINT of G st a*c = b*d holds a = c or b = d
  or c = d or a*c = c*d
proof
  let a,b,c,d be POINT of G;
  assume that
A1: a*c = b*d & not a = c & not b = d and
A2: not c = d;
  c on a*c & d on a*c by A1,Th16;
  hence thesis by A2,Th16;
end;
