[ns] What is the purpose of InstVar?

[nguyen hieu]

Dear friends,

I am reading chapter 13 of the book "Introduction to ns2" about trace in ns2. 
As I know, the TclObject class keeps a variable of type InstVar* to a list of 
InstVar objects in order to trace. The InstVar has three variables tracedvar_, 
name_ and next_. name_ and next_ are clear in purpose, tracedvar_ acts as a 
pointer to a TracedVar object. So I understand that the purpose of InstVar is 
to trace variables of TclObject. If tracedvar_ can be null, what is the purpose 
of InstVar now?

Hieu Nguyen