一个经典的用于编写宏的宏once-only"><link rel="stylesheet" type="text/css" href="../commonltr.css"><title>Another classic macro-writing MACRO: ONCE-ONLY（另一个经典的用于编写宏的宏：ONCE-ONLY）</title></head><body id="another-classic-macro-writing-macro-once-only另一个经典的用于编写宏的宏once-only"><main role="main"><article role="article" aria-labelledby="ariaid-title1"><h1 class="title topictitle1" id="ariaid-title1">Another classic macro-writing MACRO: <code class=