title: Calculate cumulative product / multiple
body: <p>Suppose I have a vector <code>x</code> as:</p>
  
  <pre><code>x &lt;- c(X1, X2, X3, ..., Xn)
  </code></pre>
  
  <p>I want to write a function that automatically get the result series like this:</p>
  
  <pre><code>y &lt;- c(X1, X1*X2, X1*X2*X3, ..., X1*X2*X3*...*Xn)
  </code></pre>
  
  <p>Can someone tell me how to do this in R?</p>