title: Python mirrored string function
body: <p>I'm trying to develop a function <code>mirror()</code> that takes a string and returns its mirrored string but only if the mirrored string can be represented using "mirrored" letters in the given string.</p>
  
  <p>For example,</p>
  
  <pre><code>&gt;&gt;&gt;mirror('vow')
  'wov'
  &gt;&gt;&gt;mirror('wood')
  'boow'
  &gt;&gt;&gt;mirror('bed')
  'INVALID'
  </code></pre>
  
  <p>That is, a mirrored <code>b</code> is a <code>d</code>. The letter <code>e</code> does not have any mirrored equivalent.</p>
  
  <p>So far I've started with this</p>
  
  <pre><code>def mirror(s):
      return str[::-1]
  </code></pre>
  
  <p>How can I extend this to work for whole words?</p>