def _itraj(self, value):
        """
        Reader-internal property that tracks the upcoming trajectory index. Should not be used within iterator loop.
        Parameters
        ----------
        value : int
            The upcoming trajectory index.
        """
        if value != self._selected_itraj:
            self.state.itraj = value
            # TODO: this side effect is unexpected.
            self.state.t = 0
Reader-internal property that tracks the upcoming trajectory index. Should not be used within iterator loop.
        Parameters
        ----------
        value : int
            The upcoming trajectory index.