title: The following code its giving name error
body: <pre><code>def cc(s1, s2):
      """
      &gt;&gt;&gt; cc(ravioli, ratoli)
      raoli
      """
      str(s1)
      str(s2)
      pos = 0
      s = str()
      while pos &lt;= len(s1):
          if s1[pos] == s2[:] and s1[pos] != s[:]:
              s = s + s1[pos]
              pos = pos + 1
          else:
              s = s
              pos = pos + 1
  
      return s
  </code></pre>
  
  <hr>
  
  <p><strong>Exception raised:</strong></p>
  
  <pre><code>Traceback (most recent call last):
    File "/usr/lib/python3.4/doctest.py", line 1324, in __run
      compileflags, 1), test.globs)
    File "&lt;doctest L1G10tardor12.cc[0]&gt;", line 1, in &lt;module&gt;
      cc(ravioli, ratoli)
  NameError: name 'ravioli' is not defined
  </code></pre>