type t

val print: Format.formatter -> t -> unit
val to_string: t -> string

val explain: Types.t -> Types.t -> Value.t -> t option
  (** [explain t0 t v] 
    Return a path on [v] that explains why [v] is not of type [t],
    or [None] if [v] has type [t].
    [v] is assumed to have type [t0].
  *)