title: sml in list parameter functions recursive
body: <p>list &amp; an integer n. The function should add the n-th element of the list, and return the remaining list .for example</p>
  
  <p>insertnth ([5,6,7],2,9);
  output : [5,9,6,7]</p>
  
  <p>n is the 2.
  9 is which I add the second line</p>