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(* Title: CCL/ex/nat.thy


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ID: $Id$


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Author: Martin Coen, Cambridge University Computer Laboratory


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Copyright 1993 University of Cambridge


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Programs defined over the natural numbers


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*)


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Nat = Wfd +


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consts


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not :: "i=>i"


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"#+","#*","#",


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"##","#<","#<=" :: "[i,i]=>i" (infixr 60)


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ackermann :: "[i,i]=>i"


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rules


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not_def "not(b) == if b then false else true"


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add_def "a #+ b == nrec(a,b,%x g.succ(g))"


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mult_def "a #* b == nrec(a,zero,%x g.b #+ g)"

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sub_def "a # b == 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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le_def "a #<= b == 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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lt_def "a #< b == not(b #<= a)"


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div_def "a ## b == 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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ack_def

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"ackermann(a,b) == 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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end


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