title: How to use fold in polars?
body: <p>in the API document.</p>
  <pre class="lang-py prettyprint-override"><code>df = pl.DataFrame(
      {
          &quot;a&quot;: [2, 1, 3],
          &quot;b&quot;: [1, 2, 3],
          &quot;c&quot;: [1.0, 2.0, 3.0],
      }
  )
  df.fold(lambda s1, s2: s1 + s2)
  </code></pre>
  <p>what is <code>s1</code> and <code>s2</code> refer to?</p>
  <p>In user guide:</p>
  <pre class="lang-py prettyprint-override"><code>out = df.select(
      pl.fold(acc=pl.lit(0), f=lambda acc, x: acc + x, exprs=pl.col(&quot;*&quot;)).alias(&quot;sum&quot;),
  )
  print(out)
  </code></pre>
  <p>what is <code>acc</code>?</p>