theorem elxabe (A: set) (a b: nat) (p: wff) {x: nat} (B: set x):
  $ x = a -> (b e. B <-> p) $ >
  $ a, b e. X\ x e. A, B <-> a e. A /\ p $;
    
      
        | Step | Hyp | Ref | Expression | 
        
          | 1 | 
           | 
          hyp h | 
          x = a -> (b e. B <-> p)  | 
        
        
          | 2 | 
          1 | 
          anwr | 
          T. /\ x = a -> (b e. B <-> p)  | 
        
        
          | 3 | 
          2 | 
          elxabed | 
          T. -> (a, b e. X\ x e. A, B <-> a e. A /\ p)  | 
        
        
          | 4 | 
          3 | 
          trud | 
          a, b e. X\ x e. A, B <-> a e. A /\ p  | 
        
      
    
    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)