01.hs:45:15: error:
    • Could not deduce (List l1) arising from a use of ‘nil’
      from the context: List l
        bound by the instance declaration at 01.hs:44:10-30
      Possible fix:
        add (List l1) to the context of
          an expression type signature:
            l1 a1
    • In the second argument of ‘SL’, namely ‘(nil :: l a)’
      In the expression: SL 0 (nil :: l a)
      In an equation for ‘nil’: nil = SL 0 (nil :: l a)