
theorem Th7:
  for L being RelStr, a,b being Element of L holds (a is_<=_than {b
  } iff a <= b) & (a is_>=_than {b} iff b <= a)
proof
  let L be RelStr, a,b be Element of L;
A1: b in {b} by TARSKI:def 1;
  hence a is_<=_than {b} implies a <= b;
  thus a <= b implies a is_<=_than {b} by TARSKI:def 1;
  thus a is_>=_than {b} implies a >= b by A1;
  assume
A2: a >= b;
  let c be Element of L;
  thus thesis by A2,TARSKI:def 1;
end;
