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print "Content-type: text/html\n\n";

...
</SELECT>
<INPUT TYPE=button VALUE="Select All" ONCLICK="selectAll(this.form.dem
+o,true)">
<INPUT TYPE=button VALUE="Clear All" ONCLICK="selectAll(this.form.demo
+,false)">

function selectAll(cbList,bSelect) {
  for (var i=0; i<cbList.length; i++) 
    cbList[i].selected = cbList[i].checked = bSelect
}