''

def add_value(self, key, value):

    if (key is None):
        raise ValueError('Key    is    None')
    if (value is None):
        raise ValueError("Can't    put    None    in    this    dict")
    if isinstance(value, set):
        raise ValueError("Can't    put    sets    in    this    dict")
    existing = self._dict.get(key)
    if (existing is None):
        self._dict[key] = value
    elif isinstance(existing, set):
        existing.add(value)
    else:
        self._dict[key] = set([existing, value])
