theorem sappapp (F: set) (a b: nat): $ F @@ a @ b = F @ (a, b) $;
    
      
        | Step | Hyp | Ref | Expression | 
        
          | 1 | 
           | 
          theeq | 
          {a1 | b, a1 e. F @@ a} == {a2 | (a, b), a2 e. F} -> the {a1 | b, a1 e. F @@ a} = the {a2 | (a, b), a2 e. F} | 
        
        
          | 2 | 
          1 | 
          conv app | 
          {a1 | b, a1 e. F @@ a} == {a2 | (a, b), a2 e. F} -> F @@ a @ b = F @ (a, b) | 
        
        
          | 3 | 
           | 
          sapprapp | 
          F @@ a @' b == F @' (a, b)  | 
        
        
          | 4 | 
          3 | 
          conv rapp | 
          {a1 | b, a1 e. F @@ a} == {a2 | (a, b), a2 e. F} | 
        
        
          | 5 | 
          2, 4 | 
          ax_mp | 
          F @@ a @ b = F @ (a, b)  | 
        
      
    
    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)