variable existence checking

[A.V.Kuznetsov]

Hi,

Is it possible to extend macro language by checking of a (global?)
variable existence? I think, such a checking is very helpful, since
macro execution is broken when undefined variable is called.

The following syntax can be used:
if(variable)  - checking of variable value  
if(!variable)  - checking of variable negated value  
if(!!variable)  - checking of variable existence
 
I.e. new logical operator !! "existence" can be defined.

Alexey Kuznetsov