code: def _escape_token(token, alphabet):
    r
    token = token.replace(u"\\", u"\\\\").replace(u"_", u"\\u")
    ret = [c if c in alphabet and c != u"\n" else r"\%d;" % ord(c) for c in token]
    return u"".join(ret) + "_"
docs: r"""Replace characters that aren't in the alphabet and append "_" to token.
  
    Apply three transformations to the token:
      1. Replace underline character "_" with "\u", and backslash "\" with "\\".
      2. Replace characters outside of the alphabet with "\###;", where ### is the
         character's Unicode code point.
      3. Appends "_" to mark the end of a token.
  
    Args:
      token: unicode string to be escaped
      alphabet: list of all known characters
  
    Returns:
      escaped string