author  wenzelm 
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permissions  rwrr 
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(* Title: CCL/ex/Nat.thy 
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Author: Martin Coen, Cambridge University Computer Laboratory 
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Copyright 1993 University of Cambridge 
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*) 

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header {* Programs defined over the natural numbers *} 
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theory Nat 

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imports Wfd 

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begin 

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definition not :: "i=>i" 
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where "not(b) == if b then false else true" 
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definition add :: "[i,i]=>i" (infixr "#+" 60) 
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where "a #+ b == nrec(a,b,%x g. succ(g))" 
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definition mult :: "[i,i]=>i" (infixr "#*" 60) 
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where "a #* b == nrec(a,zero,%x g. b #+ g)" 
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definition sub :: "[i,i]=>i" (infixr "#" 60) 
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where 
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"a # b == 
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letrec sub x y be ncase(y,x,%yy. ncase(x,zero,%xx. sub(xx,yy))) 
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in sub(a,b)" 
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definition le :: "[i,i]=>i" (infixr "#<=" 60) 
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where 
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"a #<= b == 
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letrec le x y be ncase(x,true,%xx. ncase(y,false,%yy. le(xx,yy))) 
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in le(a,b)" 
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definition lt :: "[i,i]=>i" (infixr "#<" 60) 
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where "a #< b == not(b #<= a)" 
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definition div :: "[i,i]=>i" (infixr "##" 60) 
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where 
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"a ## b == 
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letrec div x y be if x #< y then zero else succ(div(x#y,y)) 
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in div(a,b)" 
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definition ackermann :: "[i,i]=>i" 
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where 
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"ackermann(a,b) == 
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letrec ack n m be ncase(n,succ(m),%x. 
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ncase(m,ack(x,succ(zero)),%y. ack(x,ack(succ(x),y)))) 
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in ack(a,b)" 
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lemmas nat_defs = not_def add_def mult_def sub_def le_def lt_def ackermann_def napply_def 
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lemma natBs [simp]: 

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"not(true) = false" 

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"not(false) = true" 

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"zero #+ n = n" 

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"succ(n) #+ m = succ(n #+ m)" 

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"zero #* n = zero" 

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"succ(n) #* m = m #+ (n #* m)" 

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"f^zero`a = a" 

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"f^succ(n)`a = f(f^n`a)" 

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by (simp_all add: nat_defs) 

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lemma napply_f: "n:Nat ==> f^n`f(a) = f^succ(n)`a" 

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apply (erule Nat_ind) 

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apply simp_all 

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done 

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lemma addT: "[ a:Nat; b:Nat ] ==> a #+ b : Nat" 

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apply (unfold add_def) 

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apply (tactic {* typechk_tac @{context} [] 1 *}) 
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done 
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lemma multT: "[ a:Nat; b:Nat ] ==> a #* b : Nat" 

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apply (unfold add_def mult_def) 

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apply (tactic {* typechk_tac @{context} [] 1 *}) 
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done 
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(* Defined to return zero if a<b *) 

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lemma subT: "[ a:Nat; b:Nat ] ==> a # b : Nat" 

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apply (unfold sub_def) 

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apply (tactic {* typechk_tac @{context} [] 1 *}) 
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apply (tactic {* clean_ccs_tac @{context} *}) 
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apply (erule NatPRI [THEN wfstI, THEN NatPR_wf [THEN wmap_wf, THEN wfI]]) 
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done 

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lemma leT: "[ a:Nat; b:Nat ] ==> a #<= b : Bool" 

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apply (unfold le_def) 

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apply (tactic {* typechk_tac @{context} [] 1 *}) 
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apply (tactic {* clean_ccs_tac @{context} *}) 
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apply (erule NatPRI [THEN wfstI, THEN NatPR_wf [THEN wmap_wf, THEN wfI]]) 
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done 

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lemma ltT: "[ a:Nat; b:Nat ] ==> a #< b : Bool" 

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apply (unfold not_def lt_def) 

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apply (tactic {* typechk_tac @{context} @{thms leT} 1 *}) 
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done 
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subsection {* Termination Conditions for Ackermann's Function *} 

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lemmas relI = NatPR_wf [THEN NatPR_wf [THEN lex_wf, THEN wfI]] 

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lemma "[ a:Nat; b:Nat ] ==> ackermann(a,b) : Nat" 

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apply (unfold ackermann_def) 
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apply (tactic {* gen_ccs_tac @{context} [] 1 *}) 
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apply (erule NatPRI [THEN lexI1 [THEN relI]] NatPRI [THEN lexI2 [THEN relI]])+ 
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done 

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end 