title: How to add elements of a std::set to another non-empty set?
body: <pre><code>  set&lt;int&gt; A, B;
    for (int i = 0; i &lt; 100; i++) A.insert(i);
    for (int i = 50; i &lt; 150; i++) B.insert(i);
  </code></pre>
  
  <p>How to add elements of set <code>B</code> to the set <code>A</code>?<br>
  In another words, how to implement <code>A</code> = Union(<code>A</code>, <code>B</code>)?</p>