title: insert element in a list and return the same list updated
body: <p>Hi i'm trying to insert an element in a list but it is very important from my program that the result is stored in the original list and not in a new one.<br>
  Any code that i have written or found on the internet only succeeds if you create a new list in which the end result is kept.<br>
  So my question is can anyone tell me how to define a function: insert(X,L) where X is an element and L is a list?</p>