title: Melt multiple boolean columns
body: <p>This question is an extension of the one already asked here: <a href="https://stackoverflow.com/questions/57861364/melt-multiple-boolean-columns-in-a-single-column-in-pandas">Melt multiple boolean columns in a single column in pandas</a></p>
  <p>If the data looked like:</p>
  <pre><code>| System | Windows | Linux | Mac|
  | -------- | -------------- |
  | Desktop    | True | True | False |
  | Laptop | True   | True | False
  | Mobile | True   | True | False
  | Tablet | False | True | False
  </code></pre>
  <p>where '<code>System</code>' is <code>dtype=string</code> and other columns are <code>dtype=boolean</code>. How can I make the final table look like this:</p>
  <pre><code>| OS | System |
  | --- ----- | -------------- |
  | Windows| Desktop Laptop Mobile  |
  | Linux | Desktop Laptop Mobile Tablet |
  | Mac | |
  </code></pre>
  <p>where the values in the <code>System</code> column are separated by space.</p>