type subcommand =
  | Idea
  | List
  | Move
  | Retire
  | Tag
  | Tag_add
  | Tag_colour
  | Tag_del
  | Tag_list
  | Today
  | Todo

let subcommand_of_string = function
  | "idea" -> Idea
  | "list" -> List
  | "move" -> Move
  | "retire" -> Retire
  | "tag" -> Tag
  | "tag-add" -> Tag_add
  | "tag-color" | "tag-colour" -> Tag_colour
  | "tag-del" | "tag-delete" | "tag-rm" -> Tag_del
  | "tag-list" -> Tag_list
  | "task" | "todo" -> Todo
  | "today" -> Today
  | _ -> invalid_arg "subcommand_of_string"