title: How to return to previous part of the program (Python)
body: <p>I'm novice in python and got a problem in which I would appreciate some help.
  The problem in short:</p>
  
  <ol>
  <li>ask for a string</li>
  <li>check if all letter in a predefined list</li>
  <li>if any letter is not in the list then ask for a new string, otherwise go to next step</li>
  <li>ask for a second string</li>
  <li>check again whether the second string's letters in the list</li>
  <li>if any letter is not in the list then start over with asking for a new <strong>FIRST</strong> string</li>
  </ol>
  
  <p>basicly my main question is how to go back to a previous part of my program, and it would also help if someone would write me the base of this code.
  It start like this:</p>
  
  <pre><code>list1=[a,b,c,d]
  string1=raw_input("first:")
  for i in string1:
      if i not in list1:
  </code></pre>
  
  <p>Thanks</p>