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

theorem Th37:
  a <= b implies 'not' b <= 'not' a
proof
  assume a <= b;
  then 'not' a = 'not' (b"/\"a) by Th10
    .= 'not' b "\/" 'not' a by Th36;
  hence thesis by YELLOW_0:22;
end;
