T2544.hs:19:18: error:
    • Couldn't match type ‘IxMap i0’ with ‘IxMap l’
      Expected type: IxMap l [Int]
        Actual type: IxMap i0 [Int]
      NB: ‘IxMap’ is a non-injective type family
      The type variable ‘i0’ is ambiguous
    • In the first argument of ‘BiApp’, namely ‘empty’
      In the expression: BiApp empty empty
      In an equation for ‘empty’: empty = BiApp empty empty
    • Relevant bindings include
        empty :: IxMap (l :|: r) [Int] (bound at T2544.hs:19:4)