def _add_observation(self, x_to_add, y_to_add):
self._add_observation_to_means(x_to_add, y_to_add)<EOL>self._add_observation_to_variances(x_to_add, y_to_add)<EOL>self.window_size += <NUM_LIT:1><EOL>
Add observation to window, updating means/variance efficiently.