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(* Title: HOL/Library/Quotient.thy


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ID: $Id$


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Author: Gertrud Bauer and Markus Wenzel, TU Muenchen


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*)


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header {*


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\title{Quotients}


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\author{Gertrud Bauer and Markus Wenzel}


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*}


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theory Quotient = Main:


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text {*


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Higherorder quotients are defined over partial equivalence relations


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(PERs) instead of total ones. We provide axiomatic type classes


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@{text "equiv < partial_equiv"} and a type constructor


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@{text "'a quot"} with basic operations. Note that conventional


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quotient constructions emerge as a special case. This development is


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loosely based on \cite{Slotosch:1997}.


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*}


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subsection {* Equivalence relations *}


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subsubsection {* Partial equivalence *}


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text {*


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Type class @{text partial_equiv} models partial equivalence relations


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(PERs) using the polymorphic @{text "\<sim> :: 'a => 'a => bool"} relation,


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which is required to be symmetric and transitive, but not necessarily


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reflexive.


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*}


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consts


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eqv :: "'a => 'a => bool" (infixl "\<sim>" 50)


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axclass partial_equiv < "term"


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eqv_sym [elim?]: "x \<sim> y ==> y \<sim> x"


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eqv_trans [trans]: "x \<sim> y ==> y \<sim> z ==> x \<sim> z"


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text {*


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\medskip The domain of a partial equivalence relation is the set of


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reflexive elements. Due to symmetry and transitivity this


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characterizes exactly those elements that are connected with


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\emph{any} other one.


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*}


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constdefs


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domain :: "'a::partial_equiv set"


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"domain == {x. x \<sim> x}"


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lemma domainI [intro]: "x \<sim> x ==> x \<in> domain"


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by (unfold domain_def) blast


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lemma domainD [dest]: "x \<in> domain ==> x \<sim> x"


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by (unfold domain_def) blast


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theorem domainI' [elim?]: "x \<sim> y ==> x \<in> domain"


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proof


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assume xy: "x \<sim> y"


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also from xy have "y \<sim> x" ..


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finally show "x \<sim> x" .


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qed


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subsubsection {* Equivalence on function spaces *}


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text {*


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The @{text \<sim>} relation is lifted to function spaces. It is


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important to note that this is \emph{not} the direct product, but a


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structural one corresponding to the congruence property.


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*}


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defs (overloaded)


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eqv_fun_def: "f \<sim> g == \<forall>x \<in> domain. \<forall>y \<in> domain. x \<sim> y > f x \<sim> g y"


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lemma partial_equiv_funI [intro?]:


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"(!!x y. x \<in> domain ==> y \<in> domain ==> x \<sim> y ==> f x \<sim> g y) ==> f \<sim> g"


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by (unfold eqv_fun_def) blast


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lemma partial_equiv_funD [dest?]:


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"f \<sim> g ==> x \<in> domain ==> y \<in> domain ==> x \<sim> y ==> f x \<sim> g y"


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by (unfold eqv_fun_def) blast


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text {*


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The class of partial equivalence relations is closed under function


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spaces (in \emph{both} argument positions).


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*}


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instance fun :: (partial_equiv, partial_equiv) partial_equiv


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proof intro_classes


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fix f g h :: "'a::partial_equiv => 'b::partial_equiv"


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assume fg: "f \<sim> g"


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show "g \<sim> f"


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proof


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fix x y :: 'a


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assume x: "x \<in> domain" and y: "y \<in> domain"


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assume "x \<sim> y" hence "y \<sim> x" ..


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with fg y x have "f y \<sim> g x" ..


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thus "g x \<sim> f y" ..


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qed


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assume gh: "g \<sim> h"


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show "f \<sim> h"


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proof


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fix x y :: 'a


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assume x: "x \<in> domain" and y: "y \<in> domain" and "x \<sim> y"


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with fg have "f x \<sim> g y" ..


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also from y have "y \<sim> y" ..


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with gh y y have "g y \<sim> h y" ..


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finally show "f x \<sim> h y" .


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qed


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qed


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subsubsection {* Total equivalence *}


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text {*


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The class of total equivalence relations on top of PERs. It


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coincides with the standard notion of equivalence, i.e.\


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@{text "\<sim> :: 'a => 'a => bool"} is required to be reflexive, transitive


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and symmetric.


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*}


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axclass equiv < partial_equiv


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eqv_refl [intro]: "x \<sim> x"


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text {*


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On total equivalences all elements are reflexive, and congruence


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holds unconditionally.


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*}


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theorem equiv_domain [intro]: "(x::'a::equiv) \<in> domain"


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proof


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show "x \<sim> x" ..


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qed


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theorem equiv_cong [dest?]: "f \<sim> g ==> x \<sim> y ==> f x \<sim> g (y::'a::equiv)"


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proof 


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assume "f \<sim> g"


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moreover have "x \<in> domain" ..


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moreover have "y \<in> domain" ..


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moreover assume "x \<sim> y"


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ultimately show ?thesis ..


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qed


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subsection {* Quotient types *}


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subsubsection {* General quotients and equivalence classes *}


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text {*


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The quotient type @{text "'a quot"} consists of all \emph{equivalence


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classes} over elements of the base type @{typ 'a}.


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*}


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typedef 'a quot = "{{x. a \<sim> x} a::'a. True}"


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by blast


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lemma quotI [intro]: "{x. a \<sim> x} \<in> quot"


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by (unfold quot_def) blast


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lemma quotE [elim]: "R \<in> quot ==> (!!a. R = {x. a \<sim> x} ==> C) ==> C"


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by (unfold quot_def) blast


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text {*


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\medskip Abstracted equivalence classes are the canonical


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representation of elements of a quotient type.


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*}


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constdefs


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eqv_class :: "('a::partial_equiv) => 'a quot" ("\<lfloor>_\<rfloor>")


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"\<lfloor>a\<rfloor> == Abs_quot {x. a \<sim> x}"


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theorem quot_rep: "\<exists>a. A = \<lfloor>a\<rfloor>"

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proof (cases A)


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fix R assume R: "A = Abs_quot R"


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assume "R \<in> quot" hence "\<exists>a. R = {x. a \<sim> x}" by blast


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with R have "\<exists>a. A = Abs_quot {x. a \<sim> x}" by blast


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thus ?thesis by (unfold eqv_class_def)

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qed


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lemma quot_cases [case_names rep, cases type: quot]:


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"(!!a. A = \<lfloor>a\<rfloor> ==> C) ==> C"


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by (insert quot_rep) blast


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subsubsection {* Equality on quotients *}


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text {*


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Equality of canonical quotient elements corresponds to the original


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relation as follows.


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*}


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theorem eqv_class_eqI [intro]: "a \<sim> b ==> \<lfloor>a\<rfloor> = \<lfloor>b\<rfloor>"


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proof 


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assume ab: "a \<sim> b"


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have "{x. a \<sim> x} = {x. b \<sim> x}"


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proof (rule Collect_cong)


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fix x show "(a \<sim> x) = (b \<sim> x)"


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proof


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from ab have "b \<sim> a" ..


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also assume "a \<sim> x"


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finally show "b \<sim> x" .


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next


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note ab


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also assume "b \<sim> x"


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finally show "a \<sim> x" .


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qed


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qed


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thus ?thesis by (simp only: eqv_class_def)


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qed


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theorem eqv_class_eqD' [dest?]: "\<lfloor>a\<rfloor> = \<lfloor>b\<rfloor> ==> a \<in> domain ==> a \<sim> b" (* FIXME [dest] would cause trouble with blast due to overloading *)


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proof (unfold eqv_class_def)


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assume "Abs_quot {x. a \<sim> x} = Abs_quot {x. b \<sim> x}"


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hence "{x. a \<sim> x} = {x. b \<sim> x}" by (simp only: Abs_quot_inject quotI)


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moreover assume "a \<in> domain" hence "a \<sim> a" ..


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ultimately have "a \<in> {x. b \<sim> x}" by blast


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hence "b \<sim> a" by blast


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thus "a \<sim> b" ..


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qed


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theorem eqv_class_eqD [dest?]: "\<lfloor>a\<rfloor> = \<lfloor>b\<rfloor> ==> a \<sim> (b::'a::equiv)" (* FIXME [dest] would cause trouble with blast due to overloading *)


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proof (rule eqv_class_eqD')


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show "a \<in> domain" ..


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qed


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lemma eqv_class_eq' [simp]: "a \<in> domain ==> (\<lfloor>a\<rfloor> = \<lfloor>b\<rfloor>) = (a \<sim> b)"


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by (insert eqv_class_eqI eqv_class_eqD') blast


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lemma eqv_class_eq [simp]: "(\<lfloor>a\<rfloor> = \<lfloor>b\<rfloor>) = (a \<sim> (b::'a::equiv))"


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by (insert eqv_class_eqI eqv_class_eqD) blast


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subsubsection {* Picking representing elements *}


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constdefs


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pick :: "'a::partial_equiv quot => 'a"


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"pick A == SOME a. A = \<lfloor>a\<rfloor>"


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theorem pick_eqv' [intro?, simp]: "a \<in> domain ==> pick \<lfloor>a\<rfloor> \<sim> a" (* FIXME [intro] !? *)


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proof (unfold pick_def)


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assume a: "a \<in> domain"


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show "(SOME x. \<lfloor>a\<rfloor> = \<lfloor>x\<rfloor>) \<sim> a"


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proof (rule someI2)


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show "\<lfloor>a\<rfloor> = \<lfloor>a\<rfloor>" ..


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fix x assume "\<lfloor>a\<rfloor> = \<lfloor>x\<rfloor>"


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hence "a \<sim> x" ..


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thus "x \<sim> a" ..


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qed


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qed


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theorem pick_eqv [intro, simp]: "pick \<lfloor>a\<rfloor> \<sim> (a::'a::equiv)"


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proof (rule pick_eqv')


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show "a \<in> domain" ..


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qed


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theorem pick_inverse: "\<lfloor>pick A\<rfloor> = (A::'a::equiv quot)"

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proof (cases A)


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fix a assume a: "A = \<lfloor>a\<rfloor>"

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hence "pick A \<sim> a" by simp

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hence "\<lfloor>pick A\<rfloor> = \<lfloor>a\<rfloor>" by simp


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with a show ?thesis by simp


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qed


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end
