title: less or less_equal using set
body: <p>We can pass a function as <code>&lt;</code>(less) operator to STL data structures such as <code>set</code>, <code>multiset</code>, <code>map</code>, <code>priority_queue</code>, ...</p>
  
  <p>Is there a problem if our function acts like <code>&lt;=</code>(less_equal)?</p>