def mate_in_region(aln, regtup):
    """ Check if mate is found within region

    Return True if mate is found within region or region is None

    Args:
        aln (:obj:`pysam.AlignedSegment`): An aligned segment
        regtup (:tuple: (chrom, start, end)): Region
    Returns:
        bool: True if mate is within region

    """
    if regtup is None:
        return True
    return aln.next_reference_id == regtup[0] and regtup[1
        ] < aln.next_reference_start < regtup[2]
