theorem prelsapp (F: set) (a b y: nat): $ b, y e. F @@ a <-> (a, b), y e. F $;
    
      
        | Step | Hyp | Ref | Expression | 
        
          | 1 | 
           | 
          bitr3 | 
          (y e. F @@ a @' b <-> b, y e. F @@ a) -> (y e. F @@ a @' b <-> (a, b), y e. F) -> (b, y e. F @@ a <-> (a, b), y e. F)  | 
        
        
          | 2 | 
           | 
          elrapp | 
          y e. F @@ a @' b <-> b, y e. F @@ a  | 
        
        
          | 3 | 
          1, 2 | 
          ax_mp | 
          (y e. F @@ a @' b <-> (a, b), y e. F) -> (b, y e. F @@ a <-> (a, b), y e. F)  | 
        
        
          | 4 | 
           | 
          bitr | 
          (y e. F @@ a @' b <-> y e. F @' (a, b)) -> (y e. F @' (a, b) <-> (a, b), y e. F) -> (y e. F @@ a @' b <-> (a, b), y e. F)  | 
        
        
          | 5 | 
           | 
          eleq2 | 
          F @@ a @' b == F @' (a, b) -> (y e. F @@ a @' b <-> y e. F @' (a, b))  | 
        
        
          | 6 | 
           | 
          sapprapp | 
          F @@ a @' b == F @' (a, b)  | 
        
        
          | 7 | 
          5, 6 | 
          ax_mp | 
          y e. F @@ a @' b <-> y e. F @' (a, b)  | 
        
        
          | 8 | 
          4, 7 | 
          ax_mp | 
          (y e. F @' (a, b) <-> (a, b), y e. F) -> (y e. F @@ a @' b <-> (a, b), y e. F)  | 
        
        
          | 9 | 
           | 
          elrapp | 
          y e. F @' (a, b) <-> (a, b), y e. F  | 
        
        
          | 10 | 
          8, 9 | 
          ax_mp | 
          y e. F @@ a @' b <-> (a, b), y e. F  | 
        
        
          | 11 | 
          3, 10 | 
          ax_mp | 
          b, y e. F @@ a <-> (a, b), y e. F  | 
        
      
    
    Axiom use
    axs_prop_calc
     (ax_1,
      ax_2,
      ax_3,
      ax_mp,
      itru),
    
axs_pred_calc
     (ax_gen,
      ax_4,
      ax_5,
      ax_6,
      ax_7,
      ax_10,
      ax_11,
      ax_12),
    
axs_set
     (elab,
      ax_8),
    
axs_the
     (theid,
      the0),
    
axs_peano
     (peano1,
      peano2,
      peano5,
      addeq,
      muleq,
      add0,
      addS,
      mul0,
      mulS)