title: Define with only 1 argument
body: <p>I'm reading external C code, and I found the following line of code:</p>
  
  <pre><code>#define __MAIN_C__
  </code></pre>
  
  <p>where <code>#define</code> is given only one "argument" (namely <code>__MAIN_C__</code>).</p>
  
  <p>Is this just a placeholder, or can it have a function?</p>