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

theorem
  'not' (a"\/"b) <= 'not' a & 'not' (a"\/"b) <= 'not' b
proof
  'not' a"/\"'not' b <= 'not' a & 'not' a"/\"'not' b <= 'not' b by YELLOW_0:23;
  hence thesis by Th36;
end;
