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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 lookup(source, keys, fallback = None):
 """Traverses the source, looking up each key. Returns None if can't find anything instead of raising an exception."""

  try:
    for key in keys:
      source = source[key]
    return source
  except (KeyError, AttributeError, TypeError):
    return fallback