Optional monitors suggestion

[Yuriy via Digitalmars-d]

bearophile, good point. What do you think of the following 
solution:
1. Declaring member variable with __monitor identifier is 
disallowed.
2. class may be defined with @__monitor attribute, in which case 
a __monitor variable will be added to it's beginning.
3. Subclass of such class may again be defined with @__monitor 
attar, in which case it will be simply ignored, as it already 
inherits __monitor from super.