src/ZF/EquivClass.thy
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(*  Title:      ZF/EquivClass.thy
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    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
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    Copyright   1994  University of Cambridge
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*)
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header{*Equivalence Relations*}
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theory EquivClass imports Trancl Perm begin
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definition
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  quotient   :: "[i,i]=>i"    (infixl "'/'/" 90)  (*set of equiv classes*)  where
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      "A//r == {r``{x} . x \<in> A}"
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definition
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  congruent  :: "[i,i=>i]=>o"  where
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      "congruent(r,b) == \<forall>y z. <y,z>:r \<longrightarrow> b(y)=b(z)"
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definition
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  congruent2 :: "[i,i,[i,i]=>i]=>o"  where
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      "congruent2(r1,r2,b) == \<forall>y1 z1 y2 z2.
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           <y1,z1>:r1 \<longrightarrow> <y2,z2>:r2 \<longrightarrow> b(y1,y2) = b(z1,z2)"
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abbreviation
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  RESPECTS ::"[i=>i, i] => o"  (infixr "respects" 80) where
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  "f respects r == congruent(r,f)"
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abbreviation
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  RESPECTS2 ::"[i=>i=>i, i] => o"  (infixr "respects2 " 80) where
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  "f respects2 r == congruent2(r,r,f)"
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    --{*Abbreviation for the common case where the relations are identical*}
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subsection{*Suppes, Theorem 70:
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    @{term r} is an equiv relation iff @{term "converse(r) O r = r"}*}
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(** first half: equiv(A,r) ==> converse(r) O r = r **)
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lemma sym_trans_comp_subset:
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    "[| sym(r); trans(r) |] ==> converse(r) O r \<subseteq> r"
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by (unfold trans_def sym_def, blast)
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lemma refl_comp_subset:
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    "[| refl(A,r); r \<subseteq> A*A |] ==> r \<subseteq> converse(r) O r"
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by (unfold refl_def, blast)
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lemma equiv_comp_eq:
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    "equiv(A,r) ==> converse(r) O r = r"
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apply (unfold equiv_def)
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apply (blast del: subsetI intro!: sym_trans_comp_subset refl_comp_subset)
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done
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(*second half*)
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lemma comp_equivI:
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    "[| converse(r) O r = r;  domain(r) = A |] ==> equiv(A,r)"
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apply (unfold equiv_def refl_def sym_def trans_def)
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apply (erule equalityE)
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apply (subgoal_tac "\<forall>x y. <x,y> \<in> r \<longrightarrow> <y,x> \<in> r", blast+)
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done
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(** Equivalence classes **)
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(*Lemma for the next result*)
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lemma equiv_class_subset:
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    "[| sym(r);  trans(r);  <a,b>: r |] ==> r``{a} \<subseteq> r``{b}"
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by (unfold trans_def sym_def, blast)
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lemma equiv_class_eq:
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    "[| equiv(A,r);  <a,b>: r |] ==> r``{a} = r``{b}"
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apply (unfold equiv_def)
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apply (safe del: subsetI intro!: equalityI equiv_class_subset)
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apply (unfold sym_def, blast)
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done
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lemma equiv_class_self:
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    "[| equiv(A,r);  a \<in> A |] ==> a \<in> r``{a}"
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by (unfold equiv_def refl_def, blast)
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(*Lemma for the next result*)
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lemma subset_equiv_class:
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    "[| equiv(A,r);  r``{b} \<subseteq> r``{a};  b \<in> A |] ==> <a,b>: r"
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by (unfold equiv_def refl_def, blast)
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lemma eq_equiv_class: "[| r``{a} = r``{b};  equiv(A,r);  b \<in> A |] ==> <a,b>: r"
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by (assumption | rule equalityD2 subset_equiv_class)+
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(*thus r``{a} = r``{b} as well*)
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lemma equiv_class_nondisjoint:
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    "[| equiv(A,r);  x: (r``{a} \<inter> r``{b}) |] ==> <a,b>: r"
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by (unfold equiv_def trans_def sym_def, blast)
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lemma equiv_type: "equiv(A,r) ==> r \<subseteq> A*A"
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by (unfold equiv_def, blast)
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lemma equiv_class_eq_iff:
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     "equiv(A,r) ==> <x,y>: r \<longleftrightarrow> r``{x} = r``{y} & x \<in> A & y \<in> A"
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by (blast intro: eq_equiv_class equiv_class_eq dest: equiv_type)
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lemma eq_equiv_class_iff:
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     "[| equiv(A,r);  x \<in> A;  y \<in> A |] ==> r``{x} = r``{y} \<longleftrightarrow> <x,y>: r"
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by (blast intro: eq_equiv_class equiv_class_eq dest: equiv_type)
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(*** Quotients ***)
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(** Introduction/elimination rules -- needed? **)
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lemma quotientI [TC]: "x \<in> A ==> r``{x}: A//r"
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apply (unfold quotient_def)
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apply (erule RepFunI)
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done
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lemma quotientE:
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    "[| X \<in> A//r;  !!x. [| X = r``{x};  x \<in> A |] ==> P |] ==> P"
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by (unfold quotient_def, blast)
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lemma Union_quotient:
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    "equiv(A,r) ==> \<Union>(A//r) = A"
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by (unfold equiv_def refl_def quotient_def, blast)
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lemma quotient_disj:
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    "[| equiv(A,r);  X \<in> A//r;  Y \<in> A//r |] ==> X=Y | (X \<inter> Y \<subseteq> 0)"
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apply (unfold quotient_def)
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apply (safe intro!: equiv_class_eq, assumption)
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apply (unfold equiv_def trans_def sym_def, blast)
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done
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subsection{*Defining Unary Operations upon Equivalence Classes*}
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(** Could have a locale with the premises equiv(A,r)  and  congruent(r,b)
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**)
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(*Conversion rule*)
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lemma UN_equiv_class:
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    "[| equiv(A,r);  b respects r;  a \<in> A |] ==> (\<Union>x\<in>r``{a}. b(x)) = b(a)"
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apply (subgoal_tac "\<forall>x \<in> r``{a}. b(x) = b(a)")
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 apply simp
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 apply (blast intro: equiv_class_self)
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apply (unfold equiv_def sym_def congruent_def, blast)
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done
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(*type checking of  @{term"\<Union>x\<in>r``{a}. b(x)"} *)
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lemma UN_equiv_class_type:
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    "[| equiv(A,r);  b respects r;  X \<in> A//r;  !!x.  x \<in> A ==> b(x) \<in> B |]
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     ==> (\<Union>x\<in>X. b(x)) \<in> B"
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apply (unfold quotient_def, safe)
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apply (simp (no_asm_simp) add: UN_equiv_class)
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done
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(*Sufficient conditions for injectiveness.  Could weaken premises!
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  major premise could be an inclusion; bcong could be !!y. y \<in> A ==> b(y):B
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*)
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lemma UN_equiv_class_inject:
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    "[| equiv(A,r);   b respects r;
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        (\<Union>x\<in>X. b(x))=(\<Union>y\<in>Y. b(y));  X \<in> A//r;  Y \<in> A//r;
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        !!x y. [| x \<in> A; y \<in> A; b(x)=b(y) |] ==> <x,y>:r |]
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     ==> X=Y"
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apply (unfold quotient_def, safe)
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apply (rule equiv_class_eq, assumption)
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apply (simp add: UN_equiv_class [of A r b])
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done
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subsection{*Defining Binary Operations upon Equivalence Classes*}
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lemma congruent2_implies_congruent:
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    "[| equiv(A,r1);  congruent2(r1,r2,b);  a \<in> A |] ==> congruent(r2,b(a))"
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by (unfold congruent_def congruent2_def equiv_def refl_def, blast)
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lemma congruent2_implies_congruent_UN:
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    "[| equiv(A1,r1);  equiv(A2,r2);  congruent2(r1,r2,b);  a \<in> A2 |] ==>
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     congruent(r1, %x1. \<Union>x2 \<in> r2``{a}. b(x1,x2))"
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apply (unfold congruent_def, safe)
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apply (frule equiv_type [THEN subsetD], assumption)
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apply clarify
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apply (simp add: UN_equiv_class congruent2_implies_congruent)
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apply (unfold congruent2_def equiv_def refl_def, blast)
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done
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lemma UN_equiv_class2:
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    "[| equiv(A1,r1);  equiv(A2,r2);  congruent2(r1,r2,b);  a1: A1;  a2: A2 |]
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     ==> (\<Union>x1 \<in> r1``{a1}. \<Union>x2 \<in> r2``{a2}. b(x1,x2)) = b(a1,a2)"
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by (simp add: UN_equiv_class congruent2_implies_congruent
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              congruent2_implies_congruent_UN)
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(*type checking*)
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lemma UN_equiv_class_type2:
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    "[| equiv(A,r);  b respects2 r;
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        X1: A//r;  X2: A//r;
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        !!x1 x2.  [| x1: A; x2: A |] ==> b(x1,x2) \<in> B
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     |] ==> (\<Union>x1\<in>X1. \<Union>x2\<in>X2. b(x1,x2)) \<in> B"
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apply (unfold quotient_def, safe)
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apply (blast intro: UN_equiv_class_type congruent2_implies_congruent_UN
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                    congruent2_implies_congruent quotientI)
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done
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(*Suggested by John Harrison -- the two subproofs may be MUCH simpler
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  than the direct proof*)
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lemma congruent2I:
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    "[|  equiv(A1,r1);  equiv(A2,r2);
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        !! y z w. [| w \<in> A2;  <y,z> \<in> r1 |] ==> b(y,w) = b(z,w);
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        !! y z w. [| w \<in> A1;  <y,z> \<in> r2 |] ==> b(w,y) = b(w,z)
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     |] ==> congruent2(r1,r2,b)"
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apply (unfold congruent2_def equiv_def refl_def, safe)
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apply (blast intro: trans)
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done
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lemma congruent2_commuteI:
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 assumes equivA: "equiv(A,r)"
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     and commute: "!! y z. [| y \<in> A;  z \<in> A |] ==> b(y,z) = b(z,y)"
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     and congt:   "!! y z w. [| w \<in> A;  <y,z>: r |] ==> b(w,y) = b(w,z)"
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 shows "b respects2 r"
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apply (insert equivA [THEN equiv_type, THEN subsetD])
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apply (rule congruent2I [OF equivA equivA])
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apply (rule commute [THEN trans])
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apply (rule_tac [3] commute [THEN trans, symmetric])
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apply (rule_tac [5] sym)
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apply (blast intro: congt)+
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done
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(*Obsolete?*)
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lemma congruent_commuteI:
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    "[| equiv(A,r);  Z \<in> A//r;
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        !!w. [| w \<in> A |] ==> congruent(r, %z. b(w,z));
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        !!x y. [| x \<in> A;  y \<in> A |] ==> b(y,x) = b(x,y)
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     |] ==> congruent(r, %w. \<Union>z\<in>Z. b(w,z))"
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apply (simp (no_asm) add: congruent_def)
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apply (safe elim!: quotientE)
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apply (frule equiv_type [THEN subsetD], assumption)
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apply (simp add: UN_equiv_class [of A r])
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apply (simp add: congruent_def)
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done
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end