T15487.hs:7:9: error:
    Ambiguous occurrence ‘null’
    It could refer to
       either ‘Prelude.null’,
              imported from ‘Prelude’ at T15487.hs:1:8-13
              (and originally defined in ‘Data.Foldable’)
           or ‘T15487.null’, defined at T15487.hs:5:1