
theorem Th9:
  for L being RelStr, x being Element of L opp, X being set holds (
  x is_<=_than X iff ~x is_>=_than X) & (x is_>=_than X iff ~x is_<=_than X)
proof
  let L be RelStr, x be Element of L opp, X be set;
  (~x)~ = ~x;
  hence thesis by Th8;
end;
