title: use getopt to obtain multiple options
body: <p>Here is my problem, I wrote a program <code>a.exe</code>, it can take several options, like <code>"r:e:m:"</code>.
  <code>r:</code> and <code>m:</code> can't appear together, which means they run totally different tasks inside <code>a.exe</code>. And <code>e:</code> is an optional option, if it shows up, then it should be with <code>r:</code>.</p>
  
  <p>How should I implement this logic with <code>getopt</code>?</p>