ert() is called.  The default
    is to output in the same character set as the input.

    Both input_charset and output_charset must have Unicode codec entries in
    the module's charset-to-codec mapping; use add_codec(charset, codecname)
    to add codecs the module does not know about.  See the codecs module's
    documentation for more information.
    """
    if body_enc == SHORTEST:
        raise ValueError('SHORTEST not allowed for body_enc')
    CHARSETS[charset] = (header_enc, body_enc, output_charset)


def add_alias(alias, canonical):
    """Add a character set alias.

    alias is the alias name, e.g. latin-1
    canonical is the character set's canonical name, e.g. iso-8859-1
    """
    ALIASES[alias] = canonical


def add_codec(charset, codecname):
    """Add a codec that map characters in the given charset to/from Unicode.

    charset is the canonical name of a character set.  codecname is the name
    of a Python codec, as appropriate for the second argument to the unicode()
    built-in, or to the encode() method of a Unicode string.
    """
    CODEC_MAP[charset] = codecname


