DefAttributes.Observertype value = unit Declare.Hook.g list Attributes.attributeThe value being stored. May be non-marshallable (typically a closure).
Register a new value and get the token used to enable and disable it.
val activate : token -> unitActivate/deactive the value attached to the token.
val deactivate : token -> unitval is_active : token -> boolDetermine if the value for the given token is active.