mod164.hs:9:5: error: [GHC-87543]
    Ambiguous occurrence ‘D1’.
    It could refer to
       either ‘Mod164_A.D1’,
              imported from ‘Mod164_A’ at mod164.hs:4:1-15
              (and originally defined at Mod164_A.hs:3:10-11),
           or ‘Mod164_B.D1’,
              imported from ‘Mod164_B’ at mod164.hs:5:1-15
              (and originally defined at Mod164_B.hs:3:10-11).