reserve L for Boolean non empty RelStr;
reserve a,b,c,d for Element of L;

theorem
  'not' a <= 'not' (a"/\"b) & 'not' b <= 'not' (a"/\"b)
proof
  'not' a <= 'not' a"\/" 'not' b & 'not' b <= 'not' a"\/" 'not' b by
YELLOW_0:22;
  hence thesis by Th36;
end;
