A TclOO question

Hello out  there,
say I have a class and 2 instances like so

oo::class create MyClass {
    constructor {someVal} {
        my variable myVar
        set myVar $someVal
    }
    method get {} {
        my variable myVar
       return $myVar
   }
    method set {val} {
        my variable myVar
        set myVar $val
    }
}

MyClass create obj1 33
MyClass create obj2 99

How can I set a trace on 'myVar' of 'obj2'? 
I found an example on the doc of the 'my' command but it sets a trace
on a global variable during which an object is invoked - which doesn't
fit my situation
Any help will be greatly appreciated
Helmut