T2544.hs:15:18:
    Could not deduce (IxMap i0 ~ IxMap l)
    from the context (Ix l, Ix r)
      bound by the instance declaration at T2544.hs:13:10-37
    NB: `IxMap' is a type function, and may not be injective
    Expected type: IxMap l [Int]
      Actual type: IxMap i0 [Int]
    In the first argument of `BiApp', namely `empty'
    In the expression: BiApp empty empty
    In an equation for `empty': empty = BiApp empty empty

T2544.hs:15:24:
    Could not deduce (IxMap i1 ~ IxMap r)
    from the context (Ix l, Ix r)
      bound by the instance declaration at T2544.hs:13:10-37
    NB: `IxMap' is a type function, and may not be injective
    Expected type: IxMap r [Int]
      Actual type: IxMap i1 [Int]
    In the second argument of `BiApp', namely `empty'
    In the expression: BiApp empty empty
    In an equation for `empty': empty = BiApp empty empty