Behavior = object
  update*: Option[proc (elem: Element; dt: float): void]
  added*: Option[proc (elem: Element): void]

proc dispatchUpdate(self: Element; dt: float): bool {...}{.
    raises: [UnpackError, KeyError, ValueError, Exception], tags: [RootEffect].}

proc add(element: Element; behavior: Behavior): void {...}{.
    raises: [Exception, UnpackError], tags: [RootEffect].}

proc behaviors(element: Element): seq[Behavior] {...}{.raises: [KeyError, ValueError],
    tags: [].}