Getting the evidence belonging to a particular timeslice.

        Parameters:
        ----------
        evidence: dict
            a dict key, value pair as {var: state_of_var_observed}
            None if no evidence

        time: int
            the evidence corresponding to the time slice

        shift: int
            shifting the evidence corresponding to the given time slice.

def _get_evidence(self, evidence_dict, time_slice, shift):
        """
        Getting the evidence belonging to a particular timeslice.

        Parameters:
        ----------
        evidence: dict
            a dict key, value pair as {var: state_of_var_observed}
            None if no evidence

        time: int
            the evidence corresponding to the time slice

        shift: int
            shifting the evidence corresponding to the given time slice.
        """
        if evidence_dict:
            return {(node[0], shift): evidence_dict[node] for node in evidence_dict if node[1] == time_slice}