title: is a CSV with equal sign valid?
body: <p>I just got a CSV input file to be processed, which has an equal-sign before the first delimiting quote, and wondered if this is valid and has any purpose. Example (simplified):</p>
  
  <pre><code>"2"
  "3"
  ="4"
  </code></pre>
  
  <p>After reading some postings like <a href="https://stackoverflow.com/a/27363394/1845672">this one</a> I experimented with a CSV like this:</p>
  
  <pre><code>"2"
  "3"
  ="A1+A2"
  </code></pre>
  
  <p>and:</p>
  
  <pre><code>"2"
  "3"
  "=A1+A2"
  </code></pre>
  
  <p>It seems that both Excel and LibreOffice silently ignore the equal-sign before the quote, and nicely treat the equal-sign after the quote as the flag for a formula. However, I could not find any documentation about this.</p>
  
  <p>(For Excel, this CSV needs to be saved with the .txt extension, and opened with control-O)</p>
  
  <p>I am inclined to call the CSV with equal-sign before the open quote as an error that is easy to deal with when reading this file, but still wondering if there is more to say about this.</p>