Nebular spectrum of SN 2018ibb (black) at t_max+637.3 days and its decomposition into a CSM interaction (blue) and PISN component (red). This decomposition reveals that the shape of the spectrum in the blue is similar to the pseudo-continuum seen in interaction-powered SNe (Type Ia-CSM, Ibn, Icn and IIn). The emission lines in the blue arise either from CSM interaction or from material in the CSM shell excited by the SN light. The dotted vertical lines indicate the location of strong galaxy emission lines.