DATA Type
  | Var
      name  : Ident
  | Internal
      inter : Internal
  | Tup
      tps   : Types
  | List
      arg   : Type
  | External
      pos   : Pos
      txt   : String

DATA Internal
  | Type
      name  : Ident
      args  : TypeArgs

DATA TypeArgs
  | Subst  subst : Subst
  | Types  tps : Types

TYPE Subst = MAP {Ident} Type

DATA Scheme
  | Quant
      vars  : Idents
      inter : Internal

TYPE Types = [Type]

TYPE MaybeInternal = MAYBE Internal