All subscribe to              Partial      
          the same topic            subscriptions   
                                                    
           line                      line           
           ◉────◉────◉               ◉────◎────◉    
           a    b    c               a    b    c    
                                                    
                                                    
         1 level tree              1 level tree     
               ┌◉┐                       ┌◉┐        
               │b│                       │b│        
             ◉─┘ └─◉                   ◎─┘ └─◉      
             a     c                   a     c      
                                                    
         2 levels tree             2 levels tree    
               ┌◉┐                       ┌◉┐        
               │c│                       │c│        
            ┌◉─┘ └─◉┐                 ┌◎─┘ └─◉┐     
            │b     d│                 │b     d│     
          ◉─┘       └─◉             ◉─┘       └─◎   
          a           e             a           e