Line identification of the photospheric-phase spectrum (top) and nebular-phase spectrum (bottom). Top: The photospheric phase spectrum was fitted with the parameterised spectral synthesis code SYNOW (red curve). Most of the spectral features can be attributed to O I, Mg II, Si II, Ca II, and Fe II as seen in other SLSNe during their cool photospheric phase (Gal-Yam 2019a). In addition to the absorption lines in the SN ejecta, the photospheric phase spectrum shows conspicuous [Ca II] λλ 7291,7324, a feature that gets dominated by [O II] λλ 7320,7330 at about t_max+30 days. Bottom: The spectrum of the nebular phase consists of a blue pseudo-continuum and a series of allowed and forbidden emission lines from singly and doubly ionised oxygen, calcium, magnesium and iron. Remarkable is the presence of [O II] and [O III] in emission (as early as t_max+30 days), indicating ionising radiation from shock interactions (Sect. 5.1). SN absorption lines are indicated by dashed lines, and the locations mark the absorption trough minima (blueshifted by 8500 km s⁻¹ from their rest wavelengths). SN emission lines are indicated by solid lines; their line centres are at the velocity coordinate v = 0. Regions of strong atmospheric absorption are grey-shaded.