tcfail143.hs:30:9: error: [GHC-18872]
    • Couldn't match type ‘S Z’ with ‘Z’
        arising from a functional dependency between:
          constraint ‘MinMax (S Z) Z Z Z’ arising from a use of ‘extend’
          instance ‘MinMax a Z Z a’ at tcfail143.hs:12:10-23
    • In the expression: n1 `extend` n0
      In an equation for ‘t2’: t2 = n1 `extend` n0