Represents AssignableCancelable instances that are also
Updates the internal reference of this assignable cancelable
to the given value.
If this cancelable is already canceled, then value is
going to be canceled on assignment as well.
Cancels the unit of work represented by this reference.
Guaranteed idempotency - calling it multiple times should have the
same side-effect as calling it only once. Implementations
of this method should also be thread-safe.
true in case this cancelable hasn't been canceled,
or false otherwise.