title: Simplest way to form a union of two lists
body: <p>What is the easiest way to compare the elements of two lists say A and B with one another, and add the elements which are present in B to A only if they are not present in A?</p>
  
  <p>To illustrate,
  Take list A = {1,2,3}
  list B = {3,4,5}</p>
  
  <p>So after the operation AUB I want
  list A = {1,2,3,4,5}</p>