<body>
a = <span petal:replace="a"></span> ***
<span petal:define="a string:hello">
a = <span petal:replace="a"></span>
</span>
</body>
<body>
a = <span petal:replace="a"></span> ***
<span petal:define="a string:hello">
a = <span petal:replace="a"></span>
</span>
</body>