@type maybe() :: %ICal.Todo{
  alarms: term(),
  attachments: term(),
  attendees: term(),
  categories: term(),
  class: term(),
  comments: term(),
  completed: term(),
  contacts: term(),
  created: term(),
  custom_properties: term(),
  description: term(),
  dtstamp: term(),
  dtstart: term(),
  due: term(),
  duration: term(),
  exdates: term(),
  geo: term(),
  location: term(),
  modified: term(),
  organizer: term(),
  percent_completed: term(),
  priority: term(),
  rdates: term(),
  recurrance_id: term(),
  related_to: term(),
  request_status: term(),
  resources: term(),
  rrule: term(),
  sequence: term(),
  status: term(),
  summary: term(),
  uid: term(),
  url: term()
}

@type t() :: %ICal.Todo{
  alarms: [ICal.Alarm.t()],
  attachments: [ICal.Attachment.t()],
  attendees: [ICal.Attendee.t()],
  categories: [String.t()],
  class: nil | String.t(),
  comments: [String.t()],
  completed: nil | DateTime.t(),
  contacts: [ICal.Contact.t()],
  created: nil | DateTime.t(),
  custom_properties: ICal.custom_properties(),
  description: nil | String.t(),
  dtstamp: DateTime.t(),
  dtstart: nil | DateTime.t() | Date.t(),
  due: nil | DateTime.t() | Date.t(),
  duration: nil | ICal.Duration.t(),
  exdates: [Date.t() | DateTime.t()],
  geo: nil | ICal.geo(),
  location: nil | String.t(),
  modified: nil | DateTime.t(),
  organizer: nil | String.t(),
  percent_completed: non_neg_integer(),
  priority: non_neg_integer(),
  rdates: [Date.t() | DateTime.t() | ICal.period()],
  recurrance_id: nil | DateTime.t() | Date.t(),
  related_to: [String.t()],
  request_status: [String.t()],
  resources: [String.t()],
  rrule: nil | ICal.Recurrence.t(),
  sequence: non_neg_integer(),
  status: :need_action | :completed | :in_process | :cancelled | nil,
  summary: nil | String.t(),
  uid: String.t(),
  url: nil | String.t()
}