Let us go on modifying Zbus by including nodes and the elements connected to it one by  
      one as given in the textbook                                                       

step-1 type1 modification
Zbus= 
    0.25  

step-2 type2 modification
Zbus= 
    0.25    0.25  
    0.25    0.35  

step-3 type2 modification
Zbus= 
    0.25    0.25    0.25  
    0.25    0.35    0.25  
    0.25    0.25    0.35  

step-4 type3 modification
Zbus= 
    0.1458333    0.1041667    0.1458333  
    0.1041667    0.1458333    0.1041667  
    0.1458333    0.1041667    0.2458333  
This is the final Zbus matrix after including all the elements

step-5 type4 modification
Zbus= 
    0.1397059    0.1102941    0.125  
    0.1102941    0.1397059    0.125  
    0.125        0.125        0.175  
 
 opening a line between 2-3 is equivalent to connecting (-0.1) between bus3 bus2   
Zbus= 
    0.1458333    0.1041667    0.1458333  
    0.1041667    0.1458333    0.1041667  
    0.1458333    0.1041667    0.2458333  
(same as in step 4)