def METHOD_NAME(self, snapnum):
    filename = os.path.join(self._path, "outputs", "really_consistent_%d.list"%snapnum)
    f = open(filename)
    collist = f.readline().split()
    f.close()
    if 'Original_ID' in collist:
        read_cols = (0,collist.index('Original_ID'))
        return np.loadtxt(filename, dtype=np.int64, usecols=read_cols, unpack=True)
    else:
        raise OSError("Cannot identify Original_ID column in really_consistent_%d.list"%snapnum)