theorem b0lt (a b: nat): $ a < b <-> b0 a < b0 b $;
    
      
        | Step | Hyp | Ref | Expression | 
        
          | 1 | 
           | 
          bitr | 
          (a < b <-> 2 * a < 2 * b) -> (2 * a < 2 * b <-> b0 a < b0 b) -> (a < b <-> b0 a < b0 b)  | 
        
        
          | 2 | 
           | 
          ltmul2 | 
          0 < 2 -> (a < b <-> 2 * a < 2 * b)  | 
        
        
          | 3 | 
           | 
          d0lt2 | 
          0 < 2  | 
        
        
          | 4 | 
          2, 3 | 
          ax_mp | 
          a < b <-> 2 * a < 2 * b  | 
        
        
          | 5 | 
          1, 4 | 
          ax_mp | 
          (2 * a < 2 * b <-> b0 a < b0 b) -> (a < b <-> b0 a < b0 b)  | 
        
        
          | 6 | 
           | 
          lteq | 
          2 * a = b0 a -> 2 * b = b0 b -> (2 * a < 2 * b <-> b0 a < b0 b)  | 
        
        
          | 7 | 
           | 
          b0mul21 | 
          2 * a = b0 a  | 
        
        
          | 8 | 
          6, 7 | 
          ax_mp | 
          2 * b = b0 b -> (2 * a < 2 * b <-> b0 a < b0 b)  | 
        
        
          | 9 | 
           | 
          b0mul21 | 
          2 * b = b0 b  | 
        
        
          | 10 | 
          8, 9 | 
          ax_mp | 
          2 * a < 2 * b <-> b0 a < b0 b  | 
        
        
          | 11 | 
          5, 10 | 
          ax_mp | 
          a < b <-> b0 a < b0 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_peano
     (peano1,
      peano2,
      peano5,
      addeq,
      muleq,
      add0,
      addS,
      mul0,
      mulS)