<!--
/* この位置でスクリプトを読み込ませれば、既に <div id="a"> を読み込んでいるので、getElementById("a") がエラーにならない！ */
function c(){document.getElementById("a").style.visibility = "hidden";setTimeout(d, 500);}
function d() {document.getElementById("a").style.visibility ="visible";setTimeout(c, 500);}
function e(){document.getElementById("b").style.visibility = "hidden";setTimeout(f, 500);}
function f() {document.getElementById("b").style.visibility ="visible";setTimeout(e, 500);}
function g(){document.getElementById("c").style.visibility = "hidden";setTimeout(h, 500);}
function h() {document.getElementById("c").style.visibility ="visible";setTimeout(g, 500);}
function i(){document.getElementById("d").style.visibility = "hidden";setTimeout(j, 500);}
function j() {document.getElementById("d").style.visibility ="visible";setTimeout(i, 500);}
//-->