'Unescape an ISUPPORT parameter.
The only form of supported escape is C{\xHH}, where HH must be a valid
2-digit hexadecimal number.
@rtype: C{str}'

@classmethod
def _unescapeParamValue(cls, value):

    def _unescape():
        parts = value.split('\\x')
        (yield parts.pop(0))
        for s in parts:
            (octet, rest) = (s[:2], s[2:])
            try:
                octet = int(octet, 16)
            except ValueError:
                raise ValueError(('Invalid    hex    octet:    %r' % (octet,)))
            (yield (chr(octet) + rest))
    if ('\\x' not in value):
        return value
    return ''.join(_unescape())
