s0
               | 
               s1
            A /  \ B or else      
             /    \
            s2     s1         
    else  / | \ A    \
         / B|  \      .      
       s1   |   s2     .                
      /     |                     
     .      s3  
         A /   \ B or else                        
          /     \
        s4       s1      
        /          \
       .            .