"ph codeph">ONCE-ONLY</code>（另一个经典的用于编写宏的宏：<code class="ph codeph">ONCE-ONLY</code>）</h1><div class="body"><p class="p">Another classic macro-writing macro is <code class="ph codeph">once-only</code>, which is used to
generate code that evaluates certain macro arguments once only and in
a particular order. Using once-only, you could write <code class="ph codeph">do-primes</code> almost
as simply as the original leaky version, like this:</p><p class="p">另一个经典的用�