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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def backlink(node):
 """Given a CFG with outgoing links, create incoming links."""

    seen = set()
    to_see = [node]
    while to_see:
      node = to_see.pop()
      seen.add(node)
      for succ in node.next:
        succ.prev.add(node)
        if succ not in seen:
          to_see.append(succ)