title: #define struct union and #define else still compiles any C program?
body: <p>Someone claimed in a presentation that if you add</p>
  
  <pre><code>#define struct union
  #define else
  </code></pre>
  
  <p>at the beginning of any valid <strong>C</strong> program, that program would still compile. That seems like a bold claim. Any counterexample you have in mind and prove that guy wrong?</p>