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Super-luminous supernovae (SLSNe) are rare stellar explosions whose explosion mechanism and source of immense luminosity are not well established. SN2020abjc is the longest-lasting SLSN known. Its properties are extreme comma even for slow-evolving SLSNe. Its spectrum shows no sign of interaction of the SN ejecta with a circumstellar medium and its light curve (shape and colour) rejects a central engine such as a magnetar. We are thus left with the conclusion that a long-lasting light curve equals a long diffusion time comma which again requires a very high total mass. The pair-instability-supernova (PISN) mechanism naturally provides both the energy source (a lot of Ni) and the long diffusion time (large mass). Could SN2020abjc be a PISNquestionMark We propose to obtain slitless near-IR spectroscopy with WFC3-IR and the G102 and G141 grisms to measure the amount of iron comma sulphur comma silicon and helium in the ashes of the SN progenitor. This will enable us to unveil the explosion mechanism and the progenitor of this unusual SLSN.