title: How to test if a header has already been included before
body: <p>I'm making a project, and I get stunned with a problem.</p>
  
  <p>I have 3 libraries.h that includes another special library, definitions.h, but in my Main module, i want to include all the libraries just one time, I mean, I want to test if the library <code>definitions.h</code> has already been included, and include it or not depending on the result.</p>
  
  <p>Something like</p>
  
  <pre><code>If !(#include"definitions.h")
  (#include"definitions.h")
  </code></pre>