Electron density diagnostics. The [O II] λ3729/3727 ratio and the lower limit on the N IV] λ1483/1486 ratio, as well as the inferred electron densities, are reported as measured for GS-z9-0. The expected behaviour of the two line ratios as a function of density are depicted by the solid lines for T_e = 2·10⁴ K (with shaded regions spanning T_e = 1−3·10⁴ K). The lower limit on N IV] λ1483/1486 suggests no significant contribution from very high-density regions (n_e≳10⁴ cm⁻³), in broad agreement with the value inferred from the optical [O II] λ3729/3727 diagnostics.