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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 decode(tokens):
 """Decode a list of tokens to a unicode string. Args: tokens: a list of Unicode strings Returns: a unicode string """

  token_is_alnum = [t[0] in _ALPHANUMERIC_CHAR_SET for t in tokens]
  ret = []
  for i, token in enumerate(tokens):
    if i > 0 and token_is_alnum[i - 1] and token_is_alnum[i]:
      ret.append(u" ")
    ret.append(token)
  return "".join(ret)