object
  method constraint_ : Type.constraint_ -> 'a
  method virtual crush : 'a list -> 'a
  method decl : Type.decl -> 'a
  method expr : Type.expr -> 'a
  method field : Type.field -> 'a
  method gsummand : Type.gsummand -> 'a
  method poly_expr : Type.poly_expr -> 'a
  method repr : Type.repr -> 'a
  method rhs : Type.rhs -> 'a
  method summand : Type.summand -> 'a
  method tagspec : Type.tagspec -> 'a
  method variant : Type.variant -> 'a
end