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