请问一个滚动条问题

Question

<script>
var scrollspeed=100;//100px
var currentleft=1;
function myscroll(flag)
{
if(flag=="right")
{
currentleft=a1.document.body.scrollLeft;
a1.document.body.scrollLeft+=scrollspeed;
}
else
{
a1.document.body.scrollLeft-=scrollspeed;
currentleft=a1.document.body.scrollLeft;
}
}</script>
<iframe id=a1 src="about:qwertyuiopasdfghjklzxcvbnm1234567890qwertyuiopasdfghjklzxcvbnm1234567890qwertyuiopasdfghjklzxcvbnm1234567890"></iframe>
<br/>
<input type=button value=left onclick="myscroll('left')">
<input type=button value=right onclick="myscroll('right')">