smarty目录有关问题

smarty目录问题
    require("./Smarty/libs/Smarty.class.php");
    $smarty=new Smarty;
    $smarty->left_delimiter="{{";
    $smarty->right_delimiter="}}";
    $smarty->assign("main","./templates/a.html");
    $smarty->display("./templates/xx.html");
?>
为什么这里$smarty->assign("main","./templates/a.html")就不可以，
$smarty->assign("main","templates/a.html");?和$smarty->assign("main","a.html");?都可以呢，目录结构是


xx.html
nbsp;html>

{{include file=$main}}

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