Here are 4 itcl scripts that enlight the question : 

The main one : 

 package require Itcl;
 
 namespace eval itcl {
         source "MySimpleObject-spec.itcl"
         source "MySimpleObject-body-1.0.itcl"
 
         MySimpleObject my 
 
         puts [my name]
         my configure -name "Laurent"
         puts [my name]
         source "MySimpleObject-body-1.1.itcl"
         
         puts [my name]
 }

The specification of class : 

 class MySimpleObject {
         public variable name
         
         constructor { {aName "unknown"}  } {
                 set name $aName
                 return $this
         }
         
         method name {}
         
 }

A first version of the body : 

 body MySimpleObject::name {} {
         return $name
 }

A second version of the body :

 body MySimpleObject::name {} {
         return "My name is $name"
 }


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