praxis5.hs:86:28:
    Couldn't match expected type `Production v t w i'
                with actual type `[Production v0 t w0 i0]'
    Expected type: [Production v t w i]
      Actual type: [[Production v0 t w0 i0]]
    In the second argument of `wcfg', namely `productions'
    In the expression: wcfg startsymbol productions

praxis5.hs:96:23:
    Couldn't match type `t' with `v'
      `t' is a rigid type variable bound by
          the type signature for
            getProductionsFromTree :: T.Tree t -> [Production v t w i]
          at praxis5.hs:93:1
      `v' is a rigid type variable bound by
          the type signature for
            getProductionsFromTree :: T.Tree t -> [Production v t w i]
          at praxis5.hs:93:1
    In the first argument of `production', namely `lhs'
    In the expression: production lhs rhs 0 0
    In the expression: [production lhs rhs 0 0]

praxis5.hs:100:42:
    Couldn't match type `t' with `v'
      `t' is a rigid type variable bound by
          the type signature for determine :: T.Tree t -> Either v t
          at praxis5.hs:99:1
      `v' is a rigid type variable bound by
          the type signature for determine :: T.Tree t -> Either v t
          at praxis5.hs:99:1
    Expected type: T.Tree v
      Actual type: T.Tree t
    In the first argument of `T.rootLabel', namely `tree'
    In the first argument of `Left', namely `(T.rootLabel tree)'