x1 is the x2 (designation) Lagrange point for orbital system x3 under coordinate-determining convention and naming convention and calculation method x4.
Can generalize. x1 should give spacetime coordinates in some system if it is explicit. x2 can just be an ordinal (such as "first") or an indexed label (such as "L1"). Classically, x3 will probably be a two-body system such as an idealized Earth-Sol system; however, it can be generalized so as to be rather more complicated than that (which may require numerical approximations for solution rather than exact closed-form solutions).