src/HOL/Library/Multiset.thy

switched notations for pointwise and multiset order

(*  Title:      HOL/Library/Multiset.thy
    Author:     Tobias Nipkow, Markus Wenzel, Lawrence C Paulson, Norbert Voelker
*)

header {* (Finite) multisets *}

theory Multiset
imports Main
begin

subsection {* The type of multisets *}

typedef 'a multiset = "{f :: 'a => nat. finite {x. f x > 0}}"
  morphisms count Abs_multiset
proof
  show "(\<lambda>x. 0::nat) \<in> ?multiset" by simp
qed

lemmas multiset_typedef = Abs_multiset_inverse count_inverse count

abbreviation Melem :: "'a => 'a multiset => bool"  ("(_/ :# _)" [50, 51] 50) where
  "a :# M == 0 < count M a"

notation (xsymbols)
  Melem (infix "\<in>#" 50)

lemma multiset_eq_conv_count_eq:
  "M = N \<longleftrightarrow> (\<forall>a. count M a = count N a)"
  by (simp only: count_inject [symmetric] expand_fun_eq)

lemma multi_count_ext:
  "(\<And>x. count A x = count B x) \<Longrightarrow> A = B"
  using multiset_eq_conv_count_eq by auto

text {*
 \medskip Preservation of the representing set @{term multiset}.
*}

lemma const0_in_multiset:
  "(\<lambda>a. 0) \<in> multiset"
  by (simp add: multiset_def)

lemma only1_in_multiset:
  "(\<lambda>b. if b = a then n else 0) \<in> multiset"
  by (simp add: multiset_def)

lemma union_preserves_multiset:
  "M \<in> multiset \<Longrightarrow> N \<in> multiset \<Longrightarrow> (\<lambda>a. M a + N a) \<in> multiset"
  by (simp add: multiset_def)

lemma diff_preserves_multiset:
  assumes "M \<in> multiset"
  shows "(\<lambda>a. M a - N a) \<in> multiset"
proof -
  have "{x. N x < M x} \<subseteq> {x. 0 < M x}"
    by auto
  with assms show ?thesis
    by (auto simp add: multiset_def intro: finite_subset)
qed

lemma MCollect_preserves_multiset:
  assumes "M \<in> multiset"
  shows "(\<lambda>x. if P x then M x else 0) \<in> multiset"
proof -
  have "{x. (P x \<longrightarrow> 0 < M x) \<and> P x} \<subseteq> {x. 0 < M x}"
    by auto
  with assms show ?thesis
    by (auto simp add: multiset_def intro: finite_subset)
qed

lemmas in_multiset = const0_in_multiset only1_in_multiset
  union_preserves_multiset diff_preserves_multiset MCollect_preserves_multiset


subsection {* Representing multisets *}

text {* Multiset comprehension *}

definition MCollect :: "'a multiset => ('a => bool) => 'a multiset" where
  "MCollect M P = Abs_multiset (\<lambda>x. if P x then count M x else 0)"

syntax
  "_MCollect" :: "pttrn => 'a multiset => bool => 'a multiset"    ("(1{# _ :# _./ _#})")
translations
  "{#x :# M. P#}" == "CONST MCollect M (\<lambda>x. P)"


text {* Multiset enumeration *}

instantiation multiset :: (type) "{zero, plus}"
begin

definition Mempty_def:
  "0 = Abs_multiset (\<lambda>a. 0)"

abbreviation Mempty :: "'a multiset" ("{#}") where
  "Mempty \<equiv> 0"

definition union_def:
  "M + N = Abs_multiset (\<lambda>a. count M a + count N a)"

instance ..

end

definition single :: "'a => 'a multiset" where
  "single a = Abs_multiset (\<lambda>b. if b = a then 1 else 0)"

syntax
  "_multiset" :: "args => 'a multiset"    ("{#(_)#}")
translations
  "{#x, xs#}" == "{#x#} + {#xs#}"
  "{#x#}" == "CONST single x"

lemma count_empty [simp]: "count {#} a = 0"
  by (simp add: Mempty_def in_multiset multiset_typedef)

lemma count_single [simp]: "count {#b#} a = (if b = a then 1 else 0)"
  by (simp add: single_def in_multiset multiset_typedef)


subsection {* Basic operations *}

subsubsection {* Union *}

lemma count_union [simp]: "count (M + N) a = count M a + count N a"
  by (simp add: union_def in_multiset multiset_typedef)

instance multiset :: (type) cancel_comm_monoid_add proof
qed (simp_all add: multiset_eq_conv_count_eq)


subsubsection {* Difference *}

instantiation multiset :: (type) minus
begin

definition diff_def:
  "M - N = Abs_multiset (\<lambda>a. count M a - count N a)"

instance ..

end

lemma count_diff [simp]: "count (M - N) a = count M a - count N a"
  by (simp add: diff_def in_multiset multiset_typedef)

lemma diff_empty [simp]: "M - {#} = M \<and> {#} - M = {#}"
  by (simp add: Mempty_def diff_def in_multiset multiset_typedef)

lemma diff_union_inverse2 [simp]: "M + {#a#} - {#a#} = M"
  by (rule multi_count_ext)
    (auto simp del: count_single simp add: union_def diff_def in_multiset multiset_typedef)

lemma diff_cancel: "A - A = {#}"
  by (rule multi_count_ext) simp

lemma insert_DiffM:
  "x \<in># M \<Longrightarrow> {#x#} + (M - {#x#}) = M"
  by (clarsimp simp: multiset_eq_conv_count_eq)

lemma insert_DiffM2 [simp]:
  "x \<in># M \<Longrightarrow> M - {#x#} + {#x#} = M"
  by (clarsimp simp: multiset_eq_conv_count_eq)

lemma diff_right_commute:
  "(M::'a multiset) - N - Q = M - Q - N"
  by (auto simp add: multiset_eq_conv_count_eq)

lemma diff_union_swap:
  "a \<noteq> b \<Longrightarrow> M - {#a#} + {#b#} = M + {#b#} - {#a#}"
  by (auto simp add: multiset_eq_conv_count_eq)

lemma diff_union_single_conv:
  "a \<in># J \<Longrightarrow> I + J - {#a#} = I + (J - {#a#})"
  by (simp add: multiset_eq_conv_count_eq)


subsubsection {* Equality of multisets *}

lemma single_not_empty [simp]: "{#a#} \<noteq> {#} \<and> {#} \<noteq> {#a#}"
  by (simp add: multiset_eq_conv_count_eq)

lemma single_eq_single [simp]: "{#a#} = {#b#} \<longleftrightarrow> a = b"
  by (auto simp add: multiset_eq_conv_count_eq)

lemma union_eq_empty [iff]: "M + N = {#} \<longleftrightarrow> M = {#} \<and> N = {#}"
  by (auto simp add: multiset_eq_conv_count_eq)

lemma empty_eq_union [iff]: "{#} = M + N \<longleftrightarrow> M = {#} \<and> N = {#}"
  by (auto simp add: multiset_eq_conv_count_eq)

lemma multi_self_add_other_not_self [simp]: "M = M + {#x#} \<longleftrightarrow> False"
  by (auto simp add: multiset_eq_conv_count_eq)

lemma diff_single_trivial:
  "\<not> x \<in># M \<Longrightarrow> M - {#x#} = M"
  by (auto simp add: multiset_eq_conv_count_eq)

lemma diff_single_eq_union:
  "x \<in># M \<Longrightarrow> M - {#x#} = N \<longleftrightarrow> M = N + {#x#}"
  by auto

lemma union_single_eq_diff:
  "M + {#x#} = N \<Longrightarrow> M = N - {#x#}"
  by (auto dest: sym)

lemma union_single_eq_member:
  "M + {#x#} = N \<Longrightarrow> x \<in># N"
  by auto

lemma union_is_single:
  "M + N = {#a#} \<longleftrightarrow> M = {#a#} \<and> N={#} \<or> M = {#} \<and> N = {#a#}" (is "?lhs = ?rhs")
proof
  assume ?rhs then show ?lhs by auto
next
  assume ?lhs
  then have "\<And>b. count (M + N) b = (if b = a then 1 else 0)" by auto
  then have *: "\<And>b. count M b + count N b = (if b = a then 1 else 0)" by auto
  then have "count M a + count N a = 1" by auto
  then have **: "count M a = 1 \<and> count N a = 0 \<or> count M a = 0 \<and> count N a = 1"
    by auto
  from * have "\<And>b. b \<noteq> a \<Longrightarrow> count M b + count N b = 0" by auto
  then have ***: "\<And>b. b \<noteq> a \<Longrightarrow> count M b = 0 \<and> count N b = 0" by auto
  from ** and *** have
    "(\<forall>b. count M b = (if b = a then 1 else 0) \<and> count N b = 0) \<or>
      (\<forall>b. count M b = 0 \<and> count N b = (if b = a then 1 else 0))"
    by auto
  then have
    "(\<forall>b. count M b = (if b = a then 1 else 0)) \<and> (\<forall>b. count N b = 0) \<or>
      (\<forall>b. count M b = 0) \<and> (\<forall>b. count N b = (if b = a then 1 else 0))"
    by auto
  then show ?rhs by (auto simp add: multiset_eq_conv_count_eq)
qed

lemma single_is_union:
  "{#a#} = M + N \<longleftrightarrow> {#a#} = M \<and> N = {#} \<or> M = {#} \<and> {#a#} = N"
  by (auto simp add: eq_commute [of "{#a#}" "M + N"] union_is_single)

lemma add_eq_conv_diff:
  "M + {#a#} = N + {#b#} \<longleftrightarrow> M = N \<and> a = b \<or> M = N - {#a#} + {#b#} \<and> N = M - {#b#} + {#a#}"  (is "?lhs = ?rhs")
proof
  assume ?rhs then show ?lhs
  by (auto simp add: add_assoc add_commute [of "{#b#}"])
    (drule sym, simp add: add_assoc [symmetric])
next
  assume ?lhs
  show ?rhs
  proof (cases "a = b")
    case True with `?lhs` show ?thesis by simp
  next
    case False
    from `?lhs` have "a \<in># N + {#b#}" by (rule union_single_eq_member)
    with False have "a \<in># N" by auto
    moreover from `?lhs` have "M = N + {#b#} - {#a#}" by (rule union_single_eq_diff)
    moreover note False
    ultimately show ?thesis by (auto simp add: diff_right_commute [of _ "{#a#}"] diff_union_swap)
  qed
qed

lemma insert_noteq_member: 
  assumes BC: "B + {#b#} = C + {#c#}"
   and bnotc: "b \<noteq> c"
  shows "c \<in># B"
proof -
  have "c \<in># C + {#c#}" by simp
  have nc: "\<not> c \<in># {#b#}" using bnotc by simp
  then have "c \<in># B + {#b#}" using BC by simp
  then show "c \<in># B" using nc by simp
qed

lemma add_eq_conv_ex:
  "(M + {#a#} = N + {#b#}) =
    (M = N \<and> a = b \<or> (\<exists>K. M = K + {#b#} \<and> N = K + {#a#}))"
  by (auto simp add: add_eq_conv_diff)


subsubsection {* Pointwise ordering induced by count *}

instantiation multiset :: (type) ordered_ab_semigroup_add_imp_le
begin

definition less_eq_multiset :: "'a multiset \<Rightarrow> 'a multiset \<Rightarrow> bool" where
  mset_le_def: "A \<le> B \<longleftrightarrow> (\<forall>a. count A a \<le> count B a)"

definition less_multiset :: "'a multiset \<Rightarrow> 'a multiset \<Rightarrow> bool" where
  mset_less_def: "(A::'a multiset) < B \<longleftrightarrow> A \<le> B \<and> A \<noteq> B"

instance proof
qed (auto simp add: mset_le_def mset_less_def multiset_eq_conv_count_eq intro: order_trans antisym)

end

lemma mset_less_eqI:
  "(\<And>x. count A x \<le> count B x) \<Longrightarrow> A \<le> B"
  by (simp add: mset_le_def)

lemma mset_le_exists_conv:
  "(A::'a multiset) \<le> B \<longleftrightarrow> (\<exists>C. B = A + C)"
apply (unfold mset_le_def, rule iffI, rule_tac x = "B - A" in exI)
apply (auto intro: multiset_eq_conv_count_eq [THEN iffD2])
done

lemma mset_le_mono_add_right_cancel [simp]:
  "(A::'a multiset) + C \<le> B + C \<longleftrightarrow> A \<le> B"
  by (fact add_le_cancel_right)

lemma mset_le_mono_add_left_cancel [simp]:
  "C + (A::'a multiset) \<le> C + B \<longleftrightarrow> A \<le> B"
  by (fact add_le_cancel_left)

lemma mset_le_mono_add:
  "(A::'a multiset) \<le> B \<Longrightarrow> C \<le> D \<Longrightarrow> A + C \<le> B + D"
  by (fact add_mono)

lemma mset_le_add_left [simp]:
  "(A::'a multiset) \<le> A + B"
  unfolding mset_le_def by auto

lemma mset_le_add_right [simp]:
  "B \<le> (A::'a multiset) + B"
  unfolding mset_le_def by auto

lemma mset_le_single:
  "a :# B \<Longrightarrow> {#a#} \<le> B"
  by (simp add: mset_le_def)

lemma multiset_diff_union_assoc:
  "C \<le> B \<Longrightarrow> (A::'a multiset) + B - C = A + (B - C)"
  by (simp add: multiset_eq_conv_count_eq mset_le_def)

lemma mset_le_multiset_union_diff_commute:
  assumes "B \<le> A"
  shows "(A::'a multiset) - B + C = A + C - B"
proof -
  from mset_le_exists_conv [of "B" "A"] assms have "\<exists>D. A = B + D" ..
  from this obtain D where "A = B + D" ..
  then show ?thesis
    apply simp
    apply (subst add_commute)
    apply (subst multiset_diff_union_assoc)
    apply simp
    apply (simp add: diff_cancel)
    apply (subst add_assoc)
    apply (subst add_commute [of "B" _])
    apply (subst multiset_diff_union_assoc)
    apply simp
    apply (simp add: diff_cancel)
    done
qed

lemma mset_lessD: "A < B \<Longrightarrow> x \<in># A \<Longrightarrow> x \<in># B"
apply (clarsimp simp: mset_le_def mset_less_def)
apply (erule_tac x=x in allE)
apply auto
done

lemma mset_leD: "A \<le> B \<Longrightarrow> x \<in># A \<Longrightarrow> x \<in># B"
apply (clarsimp simp: mset_le_def mset_less_def)
apply (erule_tac x = x in allE)
apply auto
done
  
lemma mset_less_insertD: "(A + {#x#} < B) \<Longrightarrow> (x \<in># B \<and> A < B)"
apply (rule conjI)
 apply (simp add: mset_lessD)
apply (clarsimp simp: mset_le_def mset_less_def)
apply safe
 apply (erule_tac x = a in allE)
 apply (auto split: split_if_asm)
done

lemma mset_le_insertD: "(A + {#x#} \<le> B) \<Longrightarrow> (x \<in># B \<and> A \<le> B)"
apply (rule conjI)
 apply (simp add: mset_leD)
apply (force simp: mset_le_def mset_less_def split: split_if_asm)
done

lemma mset_less_of_empty[simp]: "A < {#} \<longleftrightarrow> False"
  by (auto simp add: mset_less_def mset_le_def multiset_eq_conv_count_eq)

lemma multi_psub_of_add_self[simp]: "A < A + {#x#}"
  by (auto simp: mset_le_def mset_less_def)

lemma multi_psub_self[simp]: "(A::'a multiset) < A = False"
  by simp

lemma mset_less_add_bothsides:
  "T + {#x#} < S + {#x#} \<Longrightarrow> T < S"
  by (fact add_less_imp_less_right)

lemma mset_less_empty_nonempty:
  "{#} < S \<longleftrightarrow> S \<noteq> {#}"
  by (auto simp: mset_le_def mset_less_def)

lemma mset_less_diff_self:
  "c \<in># B \<Longrightarrow> B - {#c#} < B"
  by (auto simp: mset_le_def mset_less_def multiset_eq_conv_count_eq)


subsubsection {* Intersection *}

instantiation multiset :: (type) semilattice_inf
begin

definition inf_multiset :: "'a multiset \<Rightarrow> 'a multiset \<Rightarrow> 'a multiset" where
  multiset_inter_def: "inf_multiset A B = A - (A - B)"

instance proof -
  have aux: "\<And>m n q :: nat. m \<le> n \<Longrightarrow> m \<le> q \<Longrightarrow> m \<le> n - (n - q)" by arith
  show "OFCLASS('a multiset, semilattice_inf_class)" proof
  qed (auto simp add: multiset_inter_def mset_le_def aux)
qed

end

abbreviation multiset_inter :: "'a multiset \<Rightarrow> 'a multiset \<Rightarrow> 'a multiset" (infixl "#\<inter>" 70) where
  "multiset_inter \<equiv> inf"

lemma multiset_inter_count:
  "count (A #\<inter> B) x = min (count A x) (count B x)"
  by (simp add: multiset_inter_def multiset_typedef)

lemma multiset_inter_single: "a \<noteq> b \<Longrightarrow> {#a#} #\<inter> {#b#} = {#}"
  by (rule multi_count_ext) (auto simp add: multiset_inter_count)

lemma multiset_union_diff_commute:
  assumes "B #\<inter> C = {#}"
  shows "A + B - C = A - C + B"
proof (rule multi_count_ext)
  fix x
  from assms have "min (count B x) (count C x) = 0"
    by (auto simp add: multiset_inter_count multiset_eq_conv_count_eq)
  then have "count B x = 0 \<or> count C x = 0"
    by auto
  then show "count (A + B - C) x = count (A - C + B) x"
    by auto
qed


subsubsection {* Comprehension (filter) *}

lemma count_MCollect [simp]:
  "count {# x:#M. P x #} a = (if P a then count M a else 0)"
  by (simp add: MCollect_def in_multiset multiset_typedef)

lemma MCollect_empty [simp]: "MCollect {#} P = {#}"
  by (rule multi_count_ext) simp

lemma MCollect_single [simp]:
  "MCollect {#x#} P = (if P x then {#x#} else {#})"
  by (rule multi_count_ext) simp

lemma MCollect_union [simp]:
  "MCollect (M + N) f = MCollect M f + MCollect N f"
  by (rule multi_count_ext) simp


subsubsection {* Set of elements *}

definition set_of :: "'a multiset => 'a set" where
  "set_of M = {x. x :# M}"

lemma set_of_empty [simp]: "set_of {#} = {}"
by (simp add: set_of_def)

lemma set_of_single [simp]: "set_of {#b#} = {b}"
by (simp add: set_of_def)

lemma set_of_union [simp]: "set_of (M + N) = set_of M \<union> set_of N"
by (auto simp add: set_of_def)

lemma set_of_eq_empty_iff [simp]: "(set_of M = {}) = (M = {#})"
by (auto simp add: set_of_def multiset_eq_conv_count_eq)

lemma mem_set_of_iff [simp]: "(x \<in> set_of M) = (x :# M)"
by (auto simp add: set_of_def)

lemma set_of_MCollect [simp]: "set_of {# x:#M. P x #} = set_of M \<inter> {x. P x}"
by (auto simp add: set_of_def)

lemma finite_set_of [iff]: "finite (set_of M)"
  using count [of M] by (simp add: multiset_def set_of_def)


subsubsection {* Size *}

instantiation multiset :: (type) size
begin

definition size_def:
  "size M = setsum (count M) (set_of M)"

instance ..

end

lemma size_empty [simp]: "size {#} = 0"
by (simp add: size_def)

lemma size_single [simp]: "size {#b#} = 1"
by (simp add: size_def)

lemma setsum_count_Int:
  "finite A ==> setsum (count N) (A \<inter> set_of N) = setsum (count N) A"
apply (induct rule: finite_induct)
 apply simp
apply (simp add: Int_insert_left set_of_def)
done

lemma size_union [simp]: "size (M + N::'a multiset) = size M + size N"
apply (unfold size_def)
apply (subgoal_tac "count (M + N) = (\<lambda>a. count M a + count N a)")
 prefer 2
 apply (rule ext, simp)
apply (simp (no_asm_simp) add: setsum_Un_nat setsum_addf setsum_count_Int)
apply (subst Int_commute)
apply (simp (no_asm_simp) add: setsum_count_Int)
done

lemma size_eq_0_iff_empty [iff]: "(size M = 0) = (M = {#})"
by (auto simp add: size_def multiset_eq_conv_count_eq)

lemma nonempty_has_size: "(S \<noteq> {#}) = (0 < size S)"
by (metis gr0I gr_implies_not0 size_empty size_eq_0_iff_empty)

lemma size_eq_Suc_imp_elem: "size M = Suc n ==> \<exists>a. a :# M"
apply (unfold size_def)
apply (drule setsum_SucD)
apply auto
done

lemma size_eq_Suc_imp_eq_union:
  assumes "size M = Suc n"
  shows "\<exists>a N. M = N + {#a#}"
proof -
  from assms obtain a where "a \<in># M"
    by (erule size_eq_Suc_imp_elem [THEN exE])
  then have "M = M - {#a#} + {#a#}" by simp
  then show ?thesis by blast
qed


subsection {* Induction and case splits *}

lemma setsum_decr:
  "finite F ==> (0::nat) < f a ==>
    setsum (f (a := f a - 1)) F = (if a\<in>F then setsum f F - 1 else setsum f F)"
apply (induct rule: finite_induct)
 apply auto
apply (drule_tac a = a in mk_disjoint_insert, auto)
done

lemma rep_multiset_induct_aux:
assumes 1: "P (\<lambda>a. (0::nat))"
  and 2: "!!f b. f \<in> multiset ==> P f ==> P (f (b := f b + 1))"
shows "\<forall>f. f \<in> multiset --> setsum f {x. f x \<noteq> 0} = n --> P f"
apply (unfold multiset_def)
apply (induct_tac n, simp, clarify)
 apply (subgoal_tac "f = (\<lambda>a.0)")
  apply simp
  apply (rule 1)
 apply (rule ext, force, clarify)
apply (frule setsum_SucD, clarify)
apply (rename_tac a)
apply (subgoal_tac "finite {x. (f (a := f a - 1)) x > 0}")
 prefer 2
 apply (rule finite_subset)
  prefer 2
  apply assumption
 apply simp
 apply blast
apply (subgoal_tac "f = (f (a := f a - 1))(a := (f (a := f a - 1)) a + 1)")
 prefer 2
 apply (rule ext)
 apply (simp (no_asm_simp))
 apply (erule ssubst, rule 2 [unfolded multiset_def], blast)
apply (erule allE, erule impE, erule_tac [2] mp, blast)
apply (simp (no_asm_simp) add: setsum_decr del: fun_upd_apply One_nat_def)
apply (subgoal_tac "{x. x \<noteq> a --> f x \<noteq> 0} = {x. f x \<noteq> 0}")
 prefer 2
 apply blast
apply (subgoal_tac "{x. x \<noteq> a \<and> f x \<noteq> 0} = {x. f x \<noteq> 0} - {a}")
 prefer 2
 apply blast
apply (simp add: le_imp_diff_is_add setsum_diff1_nat cong: conj_cong)
done

theorem rep_multiset_induct:
  "f \<in> multiset ==> P (\<lambda>a. 0) ==>
    (!!f b. f \<in> multiset ==> P f ==> P (f (b := f b + 1))) ==> P f"
using rep_multiset_induct_aux by blast

theorem multiset_induct [case_names empty add, induct type: multiset]:
assumes empty: "P {#}"
  and add: "!!M x. P M ==> P (M + {#x#})"
shows "P M"
proof -
  note defns = union_def single_def Mempty_def
  note add' = add [unfolded defns, simplified]
  have aux: "\<And>a::'a. count (Abs_multiset (\<lambda>b. if b = a then 1 else 0)) =
    (\<lambda>b. if b = a then 1 else 0)" by (simp add: Abs_multiset_inverse in_multiset) 
  show ?thesis
    apply (rule count_inverse [THEN subst])
    apply (rule count [THEN rep_multiset_induct])
     apply (rule empty [unfolded defns])
    apply (subgoal_tac "f(b := f b + 1) = (\<lambda>a. f a + (if a=b then 1 else 0))")
     prefer 2
     apply (simp add: expand_fun_eq)
    apply (erule ssubst)
    apply (erule Abs_multiset_inverse [THEN subst])
    apply (drule add')
    apply (simp add: aux)
    done
qed

lemma multi_nonempty_split: "M \<noteq> {#} \<Longrightarrow> \<exists>A a. M = A + {#a#}"
by (induct M) auto

lemma multiset_cases [cases type, case_names empty add]:
assumes em:  "M = {#} \<Longrightarrow> P"
assumes add: "\<And>N x. M = N + {#x#} \<Longrightarrow> P"
shows "P"
proof (cases "M = {#}")
  assume "M = {#}" then show ?thesis using em by simp
next
  assume "M \<noteq> {#}"
  then obtain M' m where "M = M' + {#m#}" 
    by (blast dest: multi_nonempty_split)
  then show ?thesis using add by simp
qed

lemma multi_member_split: "x \<in># M \<Longrightarrow> \<exists>A. M = A + {#x#}"
apply (cases M)
 apply simp
apply (rule_tac x="M - {#x#}" in exI, simp)
done

lemma multi_drop_mem_not_eq: "c \<in># B \<Longrightarrow> B - {#c#} \<noteq> B"
by (cases "B = {#}") (auto dest: multi_member_split)

lemma multiset_partition: "M = {# x:#M. P x #} + {# x:#M. \<not> P x #}"
apply (subst multiset_eq_conv_count_eq)
apply auto
done

lemma mset_less_size: "(A::'a multiset) < B \<Longrightarrow> size A < size B"
proof (induct A arbitrary: B)
  case (empty M)
  then have "M \<noteq> {#}" by (simp add: mset_less_empty_nonempty)
  then obtain M' x where "M = M' + {#x#}" 
    by (blast dest: multi_nonempty_split)
  then show ?case by simp
next
  case (add S x T)
  have IH: "\<And>B. S < B \<Longrightarrow> size S < size B" by fact
  have SxsubT: "S + {#x#} < T" by fact
  then have "x \<in># T" and "S < T" by (auto dest: mset_less_insertD)
  then obtain T' where T: "T = T' + {#x#}" 
    by (blast dest: multi_member_split)
  then have "S < T'" using SxsubT 
    by (blast intro: mset_less_add_bothsides)
  then have "size S < size T'" using IH by simp
  then show ?case using T by simp
qed


subsubsection {* Strong induction and subset induction for multisets *}

text {* Well-foundedness of proper subset operator: *}

text {* proper multiset subset *}

definition
  mset_less_rel :: "('a multiset * 'a multiset) set" where
  "mset_less_rel = {(A,B). A < B}"

lemma multiset_add_sub_el_shuffle: 
  assumes "c \<in># B" and "b \<noteq> c" 
  shows "B - {#c#} + {#b#} = B + {#b#} - {#c#}"
proof -
  from `c \<in># B` obtain A where B: "B = A + {#c#}" 
    by (blast dest: multi_member_split)
  have "A + {#b#} = A + {#b#} + {#c#} - {#c#}" by simp
  then have "A + {#b#} = A + {#c#} + {#b#} - {#c#}" 
    by (simp add: add_ac)
  then show ?thesis using B by simp
qed

lemma wf_mset_less_rel: "wf mset_less_rel"
apply (unfold mset_less_rel_def)
apply (rule wf_measure [THEN wf_subset, where f1=size])
apply (clarsimp simp: measure_def inv_image_def mset_less_size)
done

text {* The induction rules: *}

lemma full_multiset_induct [case_names less]:
assumes ih: "\<And>B. \<forall>(A::'a multiset). A < B \<longrightarrow> P A \<Longrightarrow> P B"
shows "P B"
apply (rule wf_mset_less_rel [THEN wf_induct])
apply (rule ih, auto simp: mset_less_rel_def)
done

lemma multi_subset_induct [consumes 2, case_names empty add]:
assumes "F \<le> A"
  and empty: "P {#}"
  and insert: "\<And>a F. a \<in># A \<Longrightarrow> P F \<Longrightarrow> P (F + {#a#})"
shows "P F"
proof -
  from `F \<le> A`
  show ?thesis
  proof (induct F)
    show "P {#}" by fact
  next
    fix x F
    assume P: "F \<le> A \<Longrightarrow> P F" and i: "F + {#x#} \<le> A"
    show "P (F + {#x#})"
    proof (rule insert)
      from i show "x \<in># A" by (auto dest: mset_le_insertD)
      from i have "F \<le> A" by (auto dest: mset_le_insertD)
      with P show "P F" .
    qed
  qed
qed


subsection {* Alternative representations *}

subsubsection {* Lists *}

primrec multiset_of :: "'a list \<Rightarrow> 'a multiset" where
  "multiset_of [] = {#}" |
  "multiset_of (a # x) = multiset_of x + {# a #}"

lemma multiset_of_zero_iff[simp]: "(multiset_of x = {#}) = (x = [])"
by (induct x) auto

lemma multiset_of_zero_iff_right[simp]: "({#} = multiset_of x) = (x = [])"
by (induct x) auto

lemma set_of_multiset_of[simp]: "set_of(multiset_of x) = set x"
by (induct x) auto

lemma mem_set_multiset_eq: "x \<in> set xs = (x :# multiset_of xs)"
by (induct xs) auto

lemma multiset_of_append [simp]:
  "multiset_of (xs @ ys) = multiset_of xs + multiset_of ys"
  by (induct xs arbitrary: ys) (auto simp: add_ac)

lemma surj_multiset_of: "surj multiset_of"
apply (unfold surj_def)
apply (rule allI)
apply (rule_tac M = y in multiset_induct)
 apply auto
apply (rule_tac x = "x # xa" in exI)
apply auto
done

lemma set_count_greater_0: "set x = {a. count (multiset_of x) a > 0}"
by (induct x) auto

lemma distinct_count_atmost_1:
  "distinct x = (! a. count (multiset_of x) a = (if a \<in> set x then 1 else 0))"
apply (induct x, simp, rule iffI, simp_all)
apply (rule conjI)
apply (simp_all add: set_of_multiset_of [THEN sym] del: set_of_multiset_of)
apply (erule_tac x = a in allE, simp, clarify)
apply (erule_tac x = aa in allE, simp)
done

lemma multiset_of_eq_setD:
  "multiset_of xs = multiset_of ys \<Longrightarrow> set xs = set ys"
by (rule) (auto simp add:multiset_eq_conv_count_eq set_count_greater_0)

lemma set_eq_iff_multiset_of_eq_distinct:
  "distinct x \<Longrightarrow> distinct y \<Longrightarrow>
    (set x = set y) = (multiset_of x = multiset_of y)"
by (auto simp: multiset_eq_conv_count_eq distinct_count_atmost_1)

lemma set_eq_iff_multiset_of_remdups_eq:
   "(set x = set y) = (multiset_of (remdups x) = multiset_of (remdups y))"
apply (rule iffI)
apply (simp add: set_eq_iff_multiset_of_eq_distinct[THEN iffD1])
apply (drule distinct_remdups [THEN distinct_remdups
      [THEN set_eq_iff_multiset_of_eq_distinct [THEN iffD2]]])
apply simp
done

lemma multiset_of_compl_union [simp]:
  "multiset_of [x\<leftarrow>xs. P x] + multiset_of [x\<leftarrow>xs. \<not>P x] = multiset_of xs"
  by (induct xs) (auto simp: add_ac)

lemma count_filter:
  "count (multiset_of xs) x = length [y \<leftarrow> xs. y = x]"
by (induct xs) auto

lemma nth_mem_multiset_of: "i < length ls \<Longrightarrow> (ls ! i) :# multiset_of ls"
apply (induct ls arbitrary: i)
 apply simp
apply (case_tac i)
 apply auto
done

lemma multiset_of_remove1: "multiset_of (remove1 a xs) = multiset_of xs - {#a#}"
by (induct xs) (auto simp add: multiset_eq_conv_count_eq)

lemma multiset_of_eq_length:
assumes "multiset_of xs = multiset_of ys"
shows "length xs = length ys"
using assms
proof (induct arbitrary: ys rule: length_induct)
  case (1 xs ys)
  show ?case
  proof (cases xs)
    case Nil with "1.prems" show ?thesis by simp
  next
    case (Cons x xs')
    note xCons = Cons
    show ?thesis
    proof (cases ys)
      case Nil
      with "1.prems" Cons show ?thesis by simp
    next
      case (Cons y ys')
      have x_in_ys: "x = y \<or> x \<in> set ys'"
      proof (cases "x = y")
        case True then show ?thesis ..
      next
        case False
        from "1.prems" [symmetric] xCons Cons have "x :# multiset_of ys' + {#y#}" by simp
        with False show ?thesis by (simp add: mem_set_multiset_eq)
      qed
      from "1.hyps" have IH: "length xs' < length xs \<longrightarrow>
        (\<forall>x. multiset_of xs' = multiset_of x \<longrightarrow> length xs' = length x)" by blast
      from "1.prems" x_in_ys Cons xCons have "multiset_of xs' = multiset_of (remove1 x (y#ys'))"
        apply -
        apply (simp add: multiset_of_remove1, simp only: add_eq_conv_diff)
        apply fastsimp
        done
      with IH xCons have IH': "length xs' = length (remove1 x (y#ys'))" by fastsimp
      from x_in_ys have "x \<noteq> y \<Longrightarrow> length ys' > 0" by auto
      with Cons xCons x_in_ys IH' show ?thesis by (auto simp add: length_remove1)
    qed
  qed
qed

text {*
  This lemma shows which properties suffice to show that a function
  @{text "f"} with @{text "f xs = ys"} behaves like sort.
*}
lemma properties_for_sort:
  "multiset_of ys = multiset_of xs \<Longrightarrow> sorted ys \<Longrightarrow> sort xs = ys"
proof (induct xs arbitrary: ys)
  case Nil then show ?case by simp
next
  case (Cons x xs)
  then have "x \<in> set ys"
    by (auto simp add:  mem_set_multiset_eq intro!: ccontr)
  with Cons.prems Cons.hyps [of "remove1 x ys"] show ?case
    by (simp add: sorted_remove1 multiset_of_remove1 insort_remove1)
qed

lemma multiset_of_remdups_le: "multiset_of (remdups xs) \<le> multiset_of xs"
  by (induct xs) (auto intro: order_trans)

lemma multiset_of_update:
  "i < length ls \<Longrightarrow> multiset_of (ls[i := v]) = multiset_of ls - {#ls ! i#} + {#v#}"
proof (induct ls arbitrary: i)
  case Nil then show ?case by simp
next
  case (Cons x xs)
  show ?case
  proof (cases i)
    case 0 then show ?thesis by simp
  next
    case (Suc i')
    with Cons show ?thesis
      apply simp
      apply (subst add_assoc)
      apply (subst add_commute [of "{#v#}" "{#x#}"])
      apply (subst add_assoc [symmetric])
      apply simp
      apply (rule mset_le_multiset_union_diff_commute)
      apply (simp add: mset_le_single nth_mem_multiset_of)
      done
  qed
qed

lemma multiset_of_swap:
  "i < length ls \<Longrightarrow> j < length ls \<Longrightarrow>
    multiset_of (ls[j := ls ! i, i := ls ! j]) = multiset_of ls"
  by (cases "i = j") (simp_all add: multiset_of_update nth_mem_multiset_of)


subsubsection {* Association lists -- including rudimentary code generation *}

definition count_of :: "('a \<times> nat) list \<Rightarrow> 'a \<Rightarrow> nat" where
  "count_of xs x = (case map_of xs x of None \<Rightarrow> 0 | Some n \<Rightarrow> n)"

lemma count_of_multiset:
  "count_of xs \<in> multiset"
proof -
  let ?A = "{x::'a. 0 < (case map_of xs x of None \<Rightarrow> 0\<Colon>nat | Some (n\<Colon>nat) \<Rightarrow> n)}"
  have "?A \<subseteq> dom (map_of xs)"
  proof
    fix x
    assume "x \<in> ?A"
    then have "0 < (case map_of xs x of None \<Rightarrow> 0\<Colon>nat | Some (n\<Colon>nat) \<Rightarrow> n)" by simp
    then have "map_of xs x \<noteq> None" by (cases "map_of xs x") auto
    then show "x \<in> dom (map_of xs)" by auto
  qed
  with finite_dom_map_of [of xs] have "finite ?A"
    by (auto intro: finite_subset)
  then show ?thesis
    by (simp add: count_of_def expand_fun_eq multiset_def)
qed

lemma count_simps [simp]:
  "count_of [] = (\<lambda>_. 0)"
  "count_of ((x, n) # xs) = (\<lambda>y. if x = y then n else count_of xs y)"
  by (simp_all add: count_of_def expand_fun_eq)

lemma count_of_empty:
  "x \<notin> fst ` set xs \<Longrightarrow> count_of xs x = 0"
  by (induct xs) (simp_all add: count_of_def)

lemma count_of_filter:
  "count_of (filter (P \<circ> fst) xs) x = (if P x then count_of xs x else 0)"
  by (induct xs) auto

definition Bag :: "('a \<times> nat) list \<Rightarrow> 'a multiset" where
  "Bag xs = Abs_multiset (count_of xs)"

code_datatype Bag

lemma count_Bag [simp, code]:
  "count (Bag xs) = count_of xs"
  by (simp add: Bag_def count_of_multiset Abs_multiset_inverse)

lemma Mempty_Bag [code]:
  "{#} = Bag []"
  by (simp add: multiset_eq_conv_count_eq)
  
lemma single_Bag [code]:
  "{#x#} = Bag [(x, 1)]"
  by (simp add: multiset_eq_conv_count_eq)

lemma MCollect_Bag [code]:
  "MCollect (Bag xs) P = Bag (filter (P \<circ> fst) xs)"
  by (simp add: multiset_eq_conv_count_eq count_of_filter)

lemma mset_less_eq_Bag [code]:
  "Bag xs \<le> A \<longleftrightarrow> (\<forall>(x, n) \<in> set xs. count_of xs x \<le> count A x)"
    (is "?lhs \<longleftrightarrow> ?rhs")
proof
  assume ?lhs then show ?rhs
    by (auto simp add: mset_le_def count_Bag)
next
  assume ?rhs
  show ?lhs
  proof (rule mset_less_eqI)
    fix x
    from `?rhs` have "count_of xs x \<le> count A x"
      by (cases "x \<in> fst ` set xs") (auto simp add: count_of_empty)
    then show "count (Bag xs) x \<le> count A x"
      by (simp add: mset_le_def count_Bag)
  qed
qed

instantiation multiset :: (eq) eq
begin

definition
  "HOL.eq A B \<longleftrightarrow> (A::'a multiset) \<le> B \<and> B \<le> A"

instance proof
qed (simp add: eq_multiset_def eq_iff)

end

definition (in term_syntax)
  bagify :: "('a\<Colon>typerep \<times> nat) list \<times> (unit \<Rightarrow> Code_Evaluation.term)
    \<Rightarrow> 'a multiset \<times> (unit \<Rightarrow> Code_Evaluation.term)" where
  [code_unfold]: "bagify xs = Code_Evaluation.valtermify Bag {\<cdot>} xs"

notation fcomp (infixl "o>" 60)
notation scomp (infixl "o\<rightarrow>" 60)

instantiation multiset :: (random) random
begin

definition
  "Quickcheck.random i = Quickcheck.random i o\<rightarrow> (\<lambda>xs. Pair (bagify xs))"

instance ..

end

no_notation fcomp (infixl "o>" 60)
no_notation scomp (infixl "o\<rightarrow>" 60)

hide (open) const bagify


subsection {* The multiset order *}

subsubsection {* Well-foundedness *}

definition mult1 :: "('a \<times> 'a) set => ('a multiset \<times> 'a multiset) set" where
  [code del]: "mult1 r = {(N, M). \<exists>a M0 K. M = M0 + {#a#} \<and> N = M0 + K \<and>
      (\<forall>b. b :# K --> (b, a) \<in> r)}"

definition mult :: "('a \<times> 'a) set => ('a multiset \<times> 'a multiset) set" where
  [code del]: "mult r = (mult1 r)\<^sup>+"

lemma not_less_empty [iff]: "(M, {#}) \<notin> mult1 r"
by (simp add: mult1_def)

lemma less_add: "(N, M0 + {#a#}) \<in> mult1 r ==>
    (\<exists>M. (M, M0) \<in> mult1 r \<and> N = M + {#a#}) \<or>
    (\<exists>K. (\<forall>b. b :# K --> (b, a) \<in> r) \<and> N = M0 + K)"
  (is "_ \<Longrightarrow> ?case1 (mult1 r) \<or> ?case2")
proof (unfold mult1_def)
  let ?r = "\<lambda>K a. \<forall>b. b :# K --> (b, a) \<in> r"
  let ?R = "\<lambda>N M. \<exists>a M0 K. M = M0 + {#a#} \<and> N = M0 + K \<and> ?r K a"
  let ?case1 = "?case1 {(N, M). ?R N M}"

  assume "(N, M0 + {#a#}) \<in> {(N, M). ?R N M}"
  then have "\<exists>a' M0' K.
      M0 + {#a#} = M0' + {#a'#} \<and> N = M0' + K \<and> ?r K a'" by simp
  then show "?case1 \<or> ?case2"
  proof (elim exE conjE)
    fix a' M0' K
    assume N: "N = M0' + K" and r: "?r K a'"
    assume "M0 + {#a#} = M0' + {#a'#}"
    then have "M0 = M0' \<and> a = a' \<or>
        (\<exists>K'. M0 = K' + {#a'#} \<and> M0' = K' + {#a#})"
      by (simp only: add_eq_conv_ex)
    then show ?thesis
    proof (elim disjE conjE exE)
      assume "M0 = M0'" "a = a'"
      with N r have "?r K a \<and> N = M0 + K" by simp
      then have ?case2 .. then show ?thesis ..
    next
      fix K'
      assume "M0' = K' + {#a#}"
      with N have n: "N = K' + K + {#a#}" by (simp add: add_ac)

      assume "M0 = K' + {#a'#}"
      with r have "?R (K' + K) M0" by blast
      with n have ?case1 by simp then show ?thesis ..
    qed
  qed
qed

lemma all_accessible: "wf r ==> \<forall>M. M \<in> acc (mult1 r)"
proof
  let ?R = "mult1 r"
  let ?W = "acc ?R"
  {
    fix M M0 a
    assume M0: "M0 \<in> ?W"
      and wf_hyp: "!!b. (b, a) \<in> r ==> (\<forall>M \<in> ?W. M + {#b#} \<in> ?W)"
      and acc_hyp: "\<forall>M. (M, M0) \<in> ?R --> M + {#a#} \<in> ?W"
    have "M0 + {#a#} \<in> ?W"
    proof (rule accI [of "M0 + {#a#}"])
      fix N
      assume "(N, M0 + {#a#}) \<in> ?R"
      then have "((\<exists>M. (M, M0) \<in> ?R \<and> N = M + {#a#}) \<or>
          (\<exists>K. (\<forall>b. b :# K --> (b, a) \<in> r) \<and> N = M0 + K))"
        by (rule less_add)
      then show "N \<in> ?W"
      proof (elim exE disjE conjE)
        fix M assume "(M, M0) \<in> ?R" and N: "N = M + {#a#}"
        from acc_hyp have "(M, M0) \<in> ?R --> M + {#a#} \<in> ?W" ..
        from this and `(M, M0) \<in> ?R` have "M + {#a#} \<in> ?W" ..
        then show "N \<in> ?W" by (simp only: N)
      next
        fix K
        assume N: "N = M0 + K"
        assume "\<forall>b. b :# K --> (b, a) \<in> r"
        then have "M0 + K \<in> ?W"
        proof (induct K)
          case empty
          from M0 show "M0 + {#} \<in> ?W" by simp
        next
          case (add K x)
          from add.prems have "(x, a) \<in> r" by simp
          with wf_hyp have "\<forall>M \<in> ?W. M + {#x#} \<in> ?W" by blast
          moreover from add have "M0 + K \<in> ?W" by simp
          ultimately have "(M0 + K) + {#x#} \<in> ?W" ..
          then show "M0 + (K + {#x#}) \<in> ?W" by (simp only: add_assoc)
        qed
        then show "N \<in> ?W" by (simp only: N)
      qed
    qed
  } note tedious_reasoning = this

  assume wf: "wf r"
  fix M
  show "M \<in> ?W"
  proof (induct M)
    show "{#} \<in> ?W"
    proof (rule accI)
      fix b assume "(b, {#}) \<in> ?R"
      with not_less_empty show "b \<in> ?W" by contradiction
    qed

    fix M a assume "M \<in> ?W"
    from wf have "\<forall>M \<in> ?W. M + {#a#} \<in> ?W"
    proof induct
      fix a
      assume r: "!!b. (b, a) \<in> r ==> (\<forall>M \<in> ?W. M + {#b#} \<in> ?W)"
      show "\<forall>M \<in> ?W. M + {#a#} \<in> ?W"
      proof
        fix M assume "M \<in> ?W"
        then show "M + {#a#} \<in> ?W"
          by (rule acc_induct) (rule tedious_reasoning [OF _ r])
      qed
    qed
    from this and `M \<in> ?W` show "M + {#a#} \<in> ?W" ..
  qed
qed

theorem wf_mult1: "wf r ==> wf (mult1 r)"
by (rule acc_wfI) (rule all_accessible)

theorem wf_mult: "wf r ==> wf (mult r)"
unfolding mult_def by (rule wf_trancl) (rule wf_mult1)


subsubsection {* Closure-free presentation *}

text {* One direction. *}

lemma mult_implies_one_step:
  "trans r ==> (M, N) \<in> mult r ==>
    \<exists>I J K. N = I + J \<and> M = I + K \<and> J \<noteq> {#} \<and>
    (\<forall>k \<in> set_of K. \<exists>j \<in> set_of J. (k, j) \<in> r)"
apply (unfold mult_def mult1_def set_of_def)
apply (erule converse_trancl_induct, clarify)
 apply (rule_tac x = M0 in exI, simp, clarify)
apply (case_tac "a :# K")
 apply (rule_tac x = I in exI)
 apply (simp (no_asm))
 apply (rule_tac x = "(K - {#a#}) + Ka" in exI)
 apply (simp (no_asm_simp) add: add_assoc [symmetric])
 apply (drule_tac f = "\<lambda>M. M - {#a#}" in arg_cong)
 apply (simp add: diff_union_single_conv)
 apply (simp (no_asm_use) add: trans_def)
 apply blast
apply (subgoal_tac "a :# I")
 apply (rule_tac x = "I - {#a#}" in exI)
 apply (rule_tac x = "J + {#a#}" in exI)
 apply (rule_tac x = "K + Ka" in exI)
 apply (rule conjI)
  apply (simp add: multiset_eq_conv_count_eq split: nat_diff_split)
 apply (rule conjI)
  apply (drule_tac f = "\<lambda>M. M - {#a#}" in arg_cong, simp)
  apply (simp add: multiset_eq_conv_count_eq split: nat_diff_split)
 apply (simp (no_asm_use) add: trans_def)
 apply blast