
theorem
  for H being non empty RelStr st H is Heyting for a,b being Element of
  H st Top H = a => b & Top H = b => a holds a = b
proof
  let H be non empty RelStr;
  assume
A1: H is Heyting;
  let a,b be Element of H;
  assume
A2: Top H = a => b;
  assume Top H = b => a;
  then
A3: b <= a by A1,Th69;
  a <= b by A1,A2,Th69;
  hence thesis by A1,A3,ORDERS_2:2;
end;
