title: next and previous for letters and symbols array
body: <p>Because </p>
  
  <pre><code>"A".next =&gt; "B"
  </code></pre>
  
  <p>and</p>
  
  <pre><code>"#".next =&gt; "$"
  </code></pre>
  
  <p>How would I find the <em>previous</em> item? So that:</p>
  
  <pre><code>"$".previous =&gt; "#"
  </code></pre>