def geom_list_2_geom(geom_list):
    """
    Join a list of md.Trajectory objects to one single trajectory


    Parameters
    ----------

    geom_list : list of md.Trajectory objects, each can have the arbitrary geom.n_frames


    Returns
    -------

    geom : of md.Trajectory object
    """
    assert isinstance(geom_list, list)
    geom = geom_list[0]
    for jgeom in geom_list[1:]:
        geom = geom.join(jgeom)
    return geom
