Args:
            exp (Experience):

def append(self, exp):
        """
        Args:
            exp (Experience):
        """
        if self._curr_size < self.max_size:
            self._assign(self._curr_pos, exp)
            self._curr_pos = (self._curr_pos + 1) % self.max_size
            self._curr_size += 1
        else:
            self._assign(self._curr_pos, exp)
            self._curr_pos = (self._curr_pos + 1) % self.max_size