theorem oddnat (p: wff): $ odd (nat p) <-> p $;
    
      
        | Step | Hyp | Ref | Expression | 
|---|
        
          | 1 |  | bitr | (odd (nat p) <-> true (nat p)) -> (true (nat p) <-> p) -> (odd (nat p) <-> p) | 
        
          | 2 |  | boolodd | bool (nat p) -> (odd (nat p) <-> true (nat p)) | 
        
          | 3 |  | boolnat | bool (nat p) | 
        
          | 4 | 2, 3 | ax_mp | odd (nat p) <-> true (nat p) | 
        
          | 5 | 1, 4 | ax_mp | (true (nat p) <-> p) -> (odd (nat p) <-> p) | 
        
          | 6 |  | truenat | true (nat p) <-> p | 
        
          | 7 | 5, 6 | ax_mp | odd (nat p) <-> 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),
    
axs_peano
     (peano1,
      peano2,
      peano5,
      addeq,
      muleq,
      add0,
      addS,
      mul0,
      mulS)