
theorem
  for H being non empty lower-bounded RelStr st H is Heyting for a,b
  being Element of H holds 'not' a >= b iff 'not' b >= a
proof
  let H be non empty lower-bounded RelStr such that
A1: H is Heyting;
  let a,b be Element of H;
A2: Bottom H >= a "/\" b iff a => Bottom H >= b by A1,Th67;
  Bottom H >= b "/\" a iff b => Bottom H >= a by A1,Th67;
  hence thesis by A1,A2,LATTICE3:15;
end;
