title: when several lists add together, finallist[1][1] cannot replace only one element
body: <p>The question is:</p>
  
  <pre><code>a = [['0','0','0']]
  b = [ ]
  b = b + a + a
  print(b) 
  </code></pre>
  
  <p>output is </p>
  
  <pre><code>[['0', '0', '0'], ['0', '0', '0']]
  </code></pre>
  
  <p>however when I type </p>
  
  <pre><code>b[0][0] = 'x'
  print(b)
  </code></pre>
  
  <p>the output is </p>
  
  <pre><code>[['x', '0', '0'], ['x', '0', '0']]
  </code></pre>
  
  <p>I am wonder how to replace only the first element in <code>b</code>, like </p>
  
  <pre><code>[['x', '0', '0'], ['0', '0', '0']]
  </code></pre>