src/ZF/Zorn.thy

verbatim

(*  Title:      ZF/Zorn.thy
    ID:         $Id$
    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
    Copyright   1994  University of Cambridge

Based upon the article
    Abrial & Laffitte, 
    Towards the Mechanization of the Proofs of Some 
    Classical Theorems of Set Theory. 

Union_in_Pow is proved in ZF.ML
*)

Zorn = OrderArith + AC + Inductive +

consts
  Subset_rel      :: i=>i
  chain, maxchain :: i=>i
  super           :: [i,i]=>i

defs
  Subset_rel_def "Subset_rel(A) == {z: A*A . EX x y. z=<x,y> & x<=y & x~=y}"

  chain_def      "chain(A)      == {F: Pow(A). ALL X:F. ALL Y:F. X<=Y | Y<=X}"
  super_def      "super(A,c)    == {d: chain(A). c<=d & c~=d}"
  maxchain_def   "maxchain(A)   == {c: chain(A). super(A,c)=0}"


(** We could make the inductive definition conditional on next: increasing(S)
    but instead we make this a side-condition of an introduction rule.  Thus
    the induction rule lets us assume that condition!  Many inductive proofs
    are therefore unconditional.
**)
consts
  "TFin" :: [i,i]=>i

inductive
  domains       "TFin(S,next)" <= "Pow(S)"
  intrs
    nextI       "[| x : TFin(S,next);  next: increasing(S) 
                |] ==> next`x : TFin(S,next)"

    Pow_UnionI  "Y : Pow(TFin(S,next)) ==> Union(Y) : TFin(S,next)"

  monos         Pow_mono
  con_defs      increasing_def
  type_intrs    "[CollectD1 RS apply_funtype, Union_in_Pow]"
  
end