'Update an item.'

def __setitem__(self, key, value):

    if (value is None):
        assert (key not in self._current)
        return
    if (key in self._current):
        self._last[key] = self._current[key].copy()
    self._current[key] = value.copy()
    self._count[key] = (self._count.get(key, 0) + 1)
