def METHOD_NAME(tmpdir, ala_mols):
    mols = ala_mols.clone()
    n_existing_atoms = mols.num_atoms()
    n_existing_residues = mols.num_residues()
    newmol = sr.mol.Molecule("dummies")
    n_to_add = 10
    editor = newmol.edit()
    # Create a residue
    editor = (
        editor.add(sr.mol.ResName("Re"))
        .renumber(sr.mol.ResNum(n_existing_residues + 1))
        .molecule()
    )
    for i in range(0, n_to_add):
        editor = (
            editor.add(sr.mol.AtomName("Re"))
            .renumber(sr.mol.AtomNum(n_existing_residues + i + 1))
            .reparent(sr.mol.ResIdx(0))
            .molecule()
        )
    mol = editor.commit()
    cursor = mol.cursor()["atomname Re"]
    # need to set the properties to the correct type...
    cursor[0]["charge"] = 1 * sr.units.mod_electron
    cursor[0]["mass"] = 1 * sr.units.g_per_mol
    for atom in cursor.atoms():
        atom["coordinates"] = sr.maths.Vector(0)
        atom["charge"] = 0 * sr.units.mod_electron
        atom["element"] = sr.mol.Element(0)
        atom["mass"] = 0 * sr.units.g_per_mol
        atom["atomtype"] = "DM"
        atom["LJ"] = sr.mm.LJParameter(
            1 * sr.units.angstrom, 0 * sr.units.kcal_per_mol
        )
    mol = cursor.molecule().commit()
    # This line is a bit janky as there isn't yet a "modern API"
    # way to do this
    mols = mols.molecules()
    mols.append(mol)
    d = tmpdir.mkdir("test_add_dummies")
    f = sr.save(mols, d.join("test"), format=["PRM7", "RST7"])
    # load to check
    mols = sr.load(f)
    mol = mols[-1]
    assert mol.num_atoms() == 10
    for atom in mol.atoms():
        assert atom.name().value() == "Re"