title: Find Closest Element in a Set
body: <p>Say I have a set, like such:</p>
  
  <pre><code>my_set = {"aaron", "cathy", "john", "stewie", "xavier"};
  </code></pre>
  
  <p>Say I want a function like such:</p>
  
  <pre><code>FindFirst(my_set, "a")      // returns an iterator pointing to "aaron"
  FindFirst(my_set, "aaron")  // returns an iterator pointing to "aaron"
  FindFirst(my_set, "bill")   // returns an iterator pointing to "cathy"
  FindFirst(my_set, "zzzzz")  // returns past-the-end iterator
  </code></pre>
  
  <p>Basically, it takes a value and returns an iterator either to that element, or the first element after it (picking the past-the-end iterator if the value provided would lie after the end of the set).</p>
  
  <p>Does any function like this exist in the standard library, or am I going to have to write one myself?</p>