@type edtf_attribute() ::
  {:unspecified, integer()}
  | {:uncertain, integer() | boolean()}
  | {:approximate, integer() | boolean()}
  | {:significant, integer()}
  | {:earlier, boolean()}
  | {:later, boolean()}

@type edtf_type() :: :date | :century | :decade | :year

@type t() ::
  %EDTF.Date{
    attributes: [{edtf_attribute(), any()}],
    level: integer(),
    type: edtf_type(),
    values: [integer() | t() | [t()]]
  }
  | nil