Methods rule_tac etc support static (Isar) contexts.
(* Title: HOL/NumberTheory/IntPrimes.thy
ID: $Id$
Author: Thomas M. Rasmussen
Copyright 2000 University of Cambridge
Changes by Jeremy Avigad, 2003/02/21:
Repaired definition of zprime_def, added "0 <= m &"
Added lemma zgcd_geq_zero
Repaired proof of zprime_imp_zrelprime
*)
header {* Divisibility and prime numbers (on integers) *}
theory IntPrimes = Primes:
text {*
The @{text dvd} relation, GCD, Euclid's extended algorithm, primes,
congruences (all on the Integers). Comparable to theory @{text
Primes}, but @{text dvd} is included here as it is not present in
main HOL. Also includes extended GCD and congruences not present in
@{text Primes}.
*}
subsection {* Definitions *}
consts
xzgcda :: "int * int * int * int * int * int * int * int => int * int * int"
recdef xzgcda
"measure ((\<lambda>(m, n, r', r, s', s, t', t). nat r)
:: int * int * int * int *int * int * int * int => nat)"
"xzgcda (m, n, r', r, s', s, t', t) =
(if r \<le> 0 then (r', s', t')
else xzgcda (m, n, r, r' mod r,
s, s' - (r' div r) * s,
t, t' - (r' div r) * t))"
constdefs
zgcd :: "int * int => int"
"zgcd == \<lambda>(x,y). int (gcd (nat (abs x), nat (abs y)))"
zprime :: "int set"
"zprime == {p. 1 < p \<and> (\<forall>m. 0 <= m & m dvd p --> m = 1 \<or> m = p)}"
xzgcd :: "int => int => int * int * int"
"xzgcd m n == xzgcda (m, n, m, n, 1, 0, 0, 1)"
zcong :: "int => int => int => bool" ("(1[_ = _] '(mod _'))")
"[a = b] (mod m) == m dvd (a - b)"
text {* \medskip @{term gcd} lemmas *}
lemma gcd_add1_eq: "gcd (m + k, k) = gcd (m + k, m)"
by (simp add: gcd_commute)
lemma gcd_diff2: "m \<le> n ==> gcd (n, n - m) = gcd (n, m)"
apply (subgoal_tac "n = m + (n - m)")
apply (erule ssubst, rule gcd_add1_eq, simp)
done
subsection {* Euclid's Algorithm and GCD *}
lemma zgcd_0 [simp]: "zgcd (m, 0) = abs m"
by (simp add: zgcd_def zabs_def)
lemma zgcd_0_left [simp]: "zgcd (0, m) = abs m"
by (simp add: zgcd_def zabs_def)
lemma zgcd_zminus [simp]: "zgcd (-m, n) = zgcd (m, n)"
by (simp add: zgcd_def)
lemma zgcd_zminus2 [simp]: "zgcd (m, -n) = zgcd (m, n)"
by (simp add: zgcd_def)
lemma zgcd_non_0: "0 < n ==> zgcd (m, n) = zgcd (n, m mod n)"
apply (frule_tac b = n and a = m in pos_mod_sign)
apply (simp del: pos_mod_sign add: zgcd_def zabs_def nat_mod_distrib)
apply (auto simp add: gcd_non_0 nat_mod_distrib [symmetric] zmod_zminus1_eq_if)
apply (frule_tac a = m in pos_mod_bound)
apply (simp del: pos_mod_bound add: nat_diff_distrib gcd_diff2 nat_le_eq_zle)
done
lemma zgcd_eq: "zgcd (m, n) = zgcd (n, m mod n)"
apply (case_tac "n = 0", simp add: DIVISION_BY_ZERO)
apply (auto simp add: linorder_neq_iff zgcd_non_0)
apply (cut_tac m = "-m" and n = "-n" in zgcd_non_0, auto)
done
lemma zgcd_1 [simp]: "zgcd (m, 1) = 1"
by (simp add: zgcd_def zabs_def)
lemma zgcd_0_1_iff [simp]: "(zgcd (0, m) = 1) = (abs m = 1)"
by (simp add: zgcd_def zabs_def)
lemma zgcd_zdvd1 [iff]: "zgcd (m, n) dvd m"
by (simp add: zgcd_def zabs_def int_dvd_iff)
lemma zgcd_zdvd2 [iff]: "zgcd (m, n) dvd n"
by (simp add: zgcd_def zabs_def int_dvd_iff)
lemma zgcd_greatest_iff: "k dvd zgcd (m, n) = (k dvd m \<and> k dvd n)"
by (simp add: zgcd_def zabs_def int_dvd_iff dvd_int_iff nat_dvd_iff)
lemma zgcd_commute: "zgcd (m, n) = zgcd (n, m)"
by (simp add: zgcd_def gcd_commute)
lemma zgcd_1_left [simp]: "zgcd (1, m) = 1"
by (simp add: zgcd_def gcd_1_left)
lemma zgcd_assoc: "zgcd (zgcd (k, m), n) = zgcd (k, zgcd (m, n))"
by (simp add: zgcd_def gcd_assoc)
lemma zgcd_left_commute: "zgcd (k, zgcd (m, n)) = zgcd (m, zgcd (k, n))"
apply (rule zgcd_commute [THEN trans])
apply (rule zgcd_assoc [THEN trans])
apply (rule zgcd_commute [THEN arg_cong])
done
lemmas zgcd_ac = zgcd_assoc zgcd_commute zgcd_left_commute
-- {* addition is an AC-operator *}
lemma zgcd_zmult_distrib2: "0 \<le> k ==> k * zgcd (m, n) = zgcd (k * m, k * n)"
by (simp del: zmult_zminus_right
add: zmult_zminus_right [symmetric] nat_mult_distrib zgcd_def zabs_def
zmult_less_0_iff gcd_mult_distrib2 [symmetric] zmult_int [symmetric])
lemma zgcd_zmult_distrib2_abs: "zgcd (k * m, k * n) = abs k * zgcd (m, n)"
by (simp add: zabs_def zgcd_zmult_distrib2)
lemma zgcd_self [simp]: "0 \<le> m ==> zgcd (m, m) = m"
by (cut_tac k = m and m = 1 and n = 1 in zgcd_zmult_distrib2, simp_all)
lemma zgcd_zmult_eq_self [simp]: "0 \<le> k ==> zgcd (k, k * n) = k"
by (cut_tac k = k and m = 1 and n = n in zgcd_zmult_distrib2, simp_all)
lemma zgcd_zmult_eq_self2 [simp]: "0 \<le> k ==> zgcd (k * n, k) = k"
by (cut_tac k = k and m = n and n = 1 in zgcd_zmult_distrib2, simp_all)
lemma zrelprime_zdvd_zmult_aux:
"zgcd (n, k) = 1 ==> k dvd m * n ==> 0 \<le> m ==> k dvd m"
apply (subgoal_tac "m = zgcd (m * n, m * k)")
apply (erule ssubst, rule zgcd_greatest_iff [THEN iffD2])
apply (simp_all add: zgcd_zmult_distrib2 [symmetric] int_0_le_mult_iff)
done
lemma zrelprime_zdvd_zmult: "zgcd (n, k) = 1 ==> k dvd m * n ==> k dvd m"
apply (case_tac "0 \<le> m")
apply (blast intro: zrelprime_zdvd_zmult_aux)
apply (subgoal_tac "k dvd -m")
apply (rule_tac [2] zrelprime_zdvd_zmult_aux, auto)
done
lemma zgcd_geq_zero: "0 <= zgcd(x,y)"
by (auto simp add: zgcd_def)
text{*This is merely a sanity check on zprime, since the previous version
denoted the empty set.*}
lemma "2 \<in> zprime"
apply (auto simp add: zprime_def)
apply (frule zdvd_imp_le, simp)
apply (auto simp add: order_le_less dvd_def)
done
lemma zprime_imp_zrelprime:
"p \<in> zprime ==> \<not> p dvd n ==> zgcd (n, p) = 1"
apply (auto simp add: zprime_def)
apply (drule_tac x = "zgcd(n, p)" in allE)
apply (auto simp add: zgcd_zdvd2 [of n p] zgcd_geq_zero)
apply (insert zgcd_zdvd1 [of n p], auto)
done
lemma zless_zprime_imp_zrelprime:
"p \<in> zprime ==> 0 < n ==> n < p ==> zgcd (n, p) = 1"
apply (erule zprime_imp_zrelprime)
apply (erule zdvd_not_zless, assumption)
done
lemma zprime_zdvd_zmult:
"0 \<le> (m::int) ==> p \<in> zprime ==> p dvd m * n ==> p dvd m \<or> p dvd n"
apply safe
apply (rule zrelprime_zdvd_zmult)
apply (rule zprime_imp_zrelprime, auto)
done
lemma zgcd_zadd_zmult [simp]: "zgcd (m + n * k, n) = zgcd (m, n)"
apply (rule zgcd_eq [THEN trans])
apply (simp add: zmod_zadd1_eq)
apply (rule zgcd_eq [symmetric])
done
lemma zgcd_zdvd_zgcd_zmult: "zgcd (m, n) dvd zgcd (k * m, n)"
apply (simp add: zgcd_greatest_iff)
apply (blast intro: zdvd_trans)
done
lemma zgcd_zmult_zdvd_zgcd:
"zgcd (k, n) = 1 ==> zgcd (k * m, n) dvd zgcd (m, n)"
apply (simp add: zgcd_greatest_iff)
apply (rule_tac n = k in zrelprime_zdvd_zmult)
prefer 2
apply (simp add: zmult_commute)
apply (subgoal_tac "zgcd (k, zgcd (k * m, n)) = zgcd (k * m, zgcd (k, n))")
apply simp
apply (simp (no_asm) add: zgcd_ac)
done
lemma zgcd_zmult_cancel: "zgcd (k, n) = 1 ==> zgcd (k * m, n) = zgcd (m, n)"
by (simp add: zgcd_def nat_abs_mult_distrib gcd_mult_cancel)
lemma zgcd_zgcd_zmult:
"zgcd (k, m) = 1 ==> zgcd (n, m) = 1 ==> zgcd (k * n, m) = 1"
by (simp add: zgcd_zmult_cancel)
lemma zdvd_iff_zgcd: "0 < m ==> (m dvd n) = (zgcd (n, m) = m)"
apply safe
apply (rule_tac [2] n = "zgcd (n, m)" in zdvd_trans)
apply (rule_tac [3] zgcd_zdvd1, simp_all)
apply (unfold dvd_def, auto)
done
subsection {* Congruences *}
lemma zcong_1 [simp]: "[a = b] (mod 1)"
by (unfold zcong_def, auto)
lemma zcong_refl [simp]: "[k = k] (mod m)"
by (unfold zcong_def, auto)
lemma zcong_sym: "[a = b] (mod m) = [b = a] (mod m)"
apply (unfold zcong_def dvd_def, auto)
apply (rule_tac [!] x = "-k" in exI, auto)
done
lemma zcong_zadd:
"[a = b] (mod m) ==> [c = d] (mod m) ==> [a + c = b + d] (mod m)"
apply (unfold zcong_def)
apply (rule_tac s = "(a - b) + (c - d)" in subst)
apply (rule_tac [2] zdvd_zadd, auto)
done
lemma zcong_zdiff:
"[a = b] (mod m) ==> [c = d] (mod m) ==> [a - c = b - d] (mod m)"
apply (unfold zcong_def)
apply (rule_tac s = "(a - b) - (c - d)" in subst)
apply (rule_tac [2] zdvd_zdiff, auto)
done
lemma zcong_trans:
"[a = b] (mod m) ==> [b = c] (mod m) ==> [a = c] (mod m)"
apply (unfold zcong_def dvd_def, auto)
apply (rule_tac x = "k + ka" in exI)
apply (simp add: zadd_ac zadd_zmult_distrib2)
done
lemma zcong_zmult:
"[a = b] (mod m) ==> [c = d] (mod m) ==> [a * c = b * d] (mod m)"
apply (rule_tac b = "b * c" in zcong_trans)
apply (unfold zcong_def)
apply (rule_tac s = "c * (a - b)" in subst)
apply (rule_tac [3] s = "b * (c - d)" in subst)
prefer 4
apply (blast intro: zdvd_zmult)
prefer 2
apply (blast intro: zdvd_zmult)
apply (simp_all add: zdiff_zmult_distrib2 zmult_commute)
done
lemma zcong_scalar: "[a = b] (mod m) ==> [a * k = b * k] (mod m)"
by (rule zcong_zmult, simp_all)
lemma zcong_scalar2: "[a = b] (mod m) ==> [k * a = k * b] (mod m)"
by (rule zcong_zmult, simp_all)
lemma zcong_zmult_self: "[a * m = b * m] (mod m)"
apply (unfold zcong_def)
apply (rule zdvd_zdiff, simp_all)
done
lemma zcong_square:
"[|p \<in> zprime; 0 < a; [a * a = 1] (mod p)|]
==> [a = 1] (mod p) \<or> [a = p - 1] (mod p)"
apply (unfold zcong_def)
apply (rule zprime_zdvd_zmult)
apply (rule_tac [3] s = "a * a - 1 + p * (1 - a)" in subst)
prefer 4
apply (simp add: zdvd_reduce)
apply (simp_all add: zdiff_zmult_distrib zmult_commute zdiff_zmult_distrib2)
done
lemma zcong_cancel:
"0 \<le> m ==>
zgcd (k, m) = 1 ==> [a * k = b * k] (mod m) = [a = b] (mod m)"
apply safe
prefer 2
apply (blast intro: zcong_scalar)
apply (case_tac "b < a")
prefer 2
apply (subst zcong_sym)
apply (unfold zcong_def)
apply (rule_tac [!] zrelprime_zdvd_zmult)
apply (simp_all add: zdiff_zmult_distrib)
apply (subgoal_tac "m dvd (-(a * k - b * k))")
apply (simp add: zminus_zdiff_eq)
apply (subst zdvd_zminus_iff, assumption)
done
lemma zcong_cancel2:
"0 \<le> m ==>
zgcd (k, m) = 1 ==> [k * a = k * b] (mod m) = [a = b] (mod m)"
by (simp add: zmult_commute zcong_cancel)
lemma zcong_zgcd_zmult_zmod:
"[a = b] (mod m) ==> [a = b] (mod n) ==> zgcd (m, n) = 1
==> [a = b] (mod m * n)"
apply (unfold zcong_def dvd_def, auto)
apply (subgoal_tac "m dvd n * ka")
apply (subgoal_tac "m dvd ka")
apply (case_tac [2] "0 \<le> ka")
prefer 3
apply (subst zdvd_zminus_iff [symmetric])
apply (rule_tac n = n in zrelprime_zdvd_zmult)
apply (simp add: zgcd_commute)
apply (simp add: zmult_commute zdvd_zminus_iff)
prefer 2
apply (rule_tac n = n in zrelprime_zdvd_zmult)
apply (simp add: zgcd_commute)
apply (simp add: zmult_commute)
apply (auto simp add: dvd_def)
done
lemma zcong_zless_imp_eq:
"0 \<le> a ==>
a < m ==> 0 \<le> b ==> b < m ==> [a = b] (mod m) ==> a = b"
apply (unfold zcong_def dvd_def, auto)
apply (drule_tac f = "\<lambda>z. z mod m" in arg_cong)
apply (cut_tac z = a and w = b in zless_linear, auto)
apply (subgoal_tac [2] "(a - b) mod m = a - b")
apply (rule_tac [3] mod_pos_pos_trivial, auto)
apply (subgoal_tac "(m + (a - b)) mod m = m + (a - b)")
apply (rule_tac [2] mod_pos_pos_trivial, auto)
done
lemma zcong_square_zless:
"p \<in> zprime ==> 0 < a ==> a < p ==>
[a * a = 1] (mod p) ==> a = 1 \<or> a = p - 1"
apply (cut_tac p = p and a = a in zcong_square)
apply (simp add: zprime_def)
apply (auto intro: zcong_zless_imp_eq)
done
lemma zcong_not:
"0 < a ==> a < m ==> 0 < b ==> b < a ==> \<not> [a = b] (mod m)"
apply (unfold zcong_def)
apply (rule zdvd_not_zless, auto)
done
lemma zcong_zless_0:
"0 \<le> a ==> a < m ==> [a = 0] (mod m) ==> a = 0"
apply (unfold zcong_def dvd_def, auto)
apply (subgoal_tac "0 < m")
apply (simp add: int_0_le_mult_iff)
apply (subgoal_tac "m * k < m * 1")
apply (drule zmult_zless_cancel1 [THEN iffD1])
apply (auto simp add: linorder_neq_iff)
done
lemma zcong_zless_unique:
"0 < m ==> (\<exists>!b. 0 \<le> b \<and> b < m \<and> [a = b] (mod m))"
apply auto
apply (subgoal_tac [2] "[b = y] (mod m)")
apply (case_tac [2] "b = 0")
apply (case_tac [3] "y = 0")
apply (auto intro: zcong_trans zcong_zless_0 zcong_zless_imp_eq order_less_le
simp add: zcong_sym)
apply (unfold zcong_def dvd_def)
apply (rule_tac x = "a mod m" in exI, auto)
apply (rule_tac x = "-(a div m)" in exI)
apply (simp add:zdiff_eq_eq eq_zdiff_eq zadd_commute)
done
lemma zcong_iff_lin: "([a = b] (mod m)) = (\<exists>k. b = a + m * k)"
apply (unfold zcong_def dvd_def, auto)
apply (rule_tac [!] x = "-k" in exI, auto)
done
lemma zgcd_zcong_zgcd:
"0 < m ==>
zgcd (a, m) = 1 ==> [a = b] (mod m) ==> zgcd (b, m) = 1"
by (auto simp add: zcong_iff_lin)
lemma zcong_zmod_aux:
"a - b = (m::int) * (a div m - b div m) + (a mod m - b mod m)"
by(simp add: zdiff_zmult_distrib2 zadd_zdiff_eq eq_zdiff_eq zadd_ac)
lemma zcong_zmod: "[a = b] (mod m) = [a mod m = b mod m] (mod m)"
apply (unfold zcong_def)
apply (rule_tac t = "a - b" in ssubst)
apply (rule_tac m = m in zcong_zmod_aux)
apply (rule trans)
apply (rule_tac [2] k = m and m = "a div m - b div m" in zdvd_reduce)
apply (simp add: zadd_commute)
done
lemma zcong_zmod_eq: "0 < m ==> [a = b] (mod m) = (a mod m = b mod m)"
apply auto
apply (rule_tac m = m in zcong_zless_imp_eq)
prefer 5
apply (subst zcong_zmod [symmetric], simp_all)
apply (unfold zcong_def dvd_def)
apply (rule_tac x = "a div m - b div m" in exI)
apply (rule_tac m1 = m in zcong_zmod_aux [THEN trans], auto)
done
lemma zcong_zminus [iff]: "[a = b] (mod -m) = [a = b] (mod m)"
by (auto simp add: zcong_def)
lemma zcong_zero [iff]: "[a = b] (mod 0) = (a = b)"
by (auto simp add: zcong_def)
lemma "[a = b] (mod m) = (a mod m = b mod m)"
apply (case_tac "m = 0", simp add: DIVISION_BY_ZERO)
apply (simp add: linorder_neq_iff)
apply (erule disjE)
prefer 2 apply (simp add: zcong_zmod_eq)
txt{*Remainding case: @{term "m<0"}*}
apply (rule_tac t = m in zminus_zminus [THEN subst])
apply (subst zcong_zminus)
apply (subst zcong_zmod_eq, arith)
apply (frule neg_mod_bound [of _ a], frule neg_mod_bound [of _ b])
apply (simp add: zmod_zminus2_eq_if del: neg_mod_bound)
done
subsection {* Modulo *}
lemma zmod_zdvd_zmod:
"0 < (m::int) ==> m dvd b ==> (a mod b mod m) = (a mod m)"
apply (unfold dvd_def, auto)
apply (subst zcong_zmod_eq [symmetric])
prefer 2
apply (subst zcong_iff_lin)
apply (rule_tac x = "k * (a div (m * k))" in exI)
apply (simp add:zmult_assoc [symmetric], assumption)
done
subsection {* Extended GCD *}
declare xzgcda.simps [simp del]
lemma xzgcd_correct_aux1:
"zgcd (r', r) = k --> 0 < r -->
(\<exists>sn tn. xzgcda (m, n, r', r, s', s, t', t) = (k, sn, tn))"
apply (rule_tac u = m and v = n and w = r' and x = r and y = s' and
z = s and aa = t' and ab = t in xzgcda.induct)
apply (subst zgcd_eq)
apply (subst xzgcda.simps, auto)
apply (case_tac "r' mod r = 0")
prefer 2
apply (frule_tac a = "r'" in pos_mod_sign, auto)
apply (rule exI)
apply (rule exI)
apply (subst xzgcda.simps, auto)
apply (simp add: zabs_def)
done
lemma xzgcd_correct_aux2:
"(\<exists>sn tn. xzgcda (m, n, r', r, s', s, t', t) = (k, sn, tn)) --> 0 < r -->
zgcd (r', r) = k"
apply (rule_tac u = m and v = n and w = r' and x = r and y = s' and
z = s and aa = t' and ab = t in xzgcda.induct)
apply (subst zgcd_eq)
apply (subst xzgcda.simps)
apply (auto simp add: linorder_not_le)
apply (case_tac "r' mod r = 0")
prefer 2
apply (frule_tac a = "r'" in pos_mod_sign, auto)
apply (erule_tac P = "xzgcda ?u = ?v" in rev_mp)
apply (subst xzgcda.simps, auto)
apply (simp add: zabs_def)
done
lemma xzgcd_correct:
"0 < n ==> (zgcd (m, n) = k) = (\<exists>s t. xzgcd m n = (k, s, t))"
apply (unfold xzgcd_def)
apply (rule iffI)
apply (rule_tac [2] xzgcd_correct_aux2 [THEN mp, THEN mp])
apply (rule xzgcd_correct_aux1 [THEN mp, THEN mp], auto)
done
text {* \medskip @{term xzgcd} linear *}
lemma xzgcda_linear_aux1:
"(a - r * b) * m + (c - r * d) * (n::int) =
(a * m + c * n) - r * (b * m + d * n)"
by (simp add: zdiff_zmult_distrib zadd_zmult_distrib2 zmult_assoc)
lemma xzgcda_linear_aux2:
"r' = s' * m + t' * n ==> r = s * m + t * n
==> (r' mod r) = (s' - (r' div r) * s) * m + (t' - (r' div r) * t) * (n::int)"
apply (rule trans)
apply (rule_tac [2] xzgcda_linear_aux1 [symmetric])
apply (simp add: eq_zdiff_eq zmult_commute)
done
lemma order_le_neq_implies_less: "(x::'a::order) \<le> y ==> x \<noteq> y ==> x < y"
by (rule iffD2 [OF order_less_le conjI])
lemma xzgcda_linear [rule_format]:
"0 < r --> xzgcda (m, n, r', r, s', s, t', t) = (rn, sn, tn) -->
r' = s' * m + t' * n --> r = s * m + t * n --> rn = sn * m + tn * n"
apply (rule_tac u = m and v = n and w = r' and x = r and y = s' and
z = s and aa = t' and ab = t in xzgcda.induct)
apply (subst xzgcda.simps)
apply (simp (no_asm))
apply (rule impI)+
apply (case_tac "r' mod r = 0")
apply (simp add: xzgcda.simps, clarify)
apply (subgoal_tac "0 < r' mod r")
apply (rule_tac [2] order_le_neq_implies_less)
apply (rule_tac [2] pos_mod_sign)
apply (cut_tac m = m and n = n and r' = r' and r = r and s' = s' and
s = s and t' = t' and t = t in xzgcda_linear_aux2, auto)
done
lemma xzgcd_linear:
"0 < n ==> xzgcd m n = (r, s, t) ==> r = s * m + t * n"
apply (unfold xzgcd_def)
apply (erule xzgcda_linear, assumption, auto)
done
lemma zgcd_ex_linear:
"0 < n ==> zgcd (m, n) = k ==> (\<exists>s t. k = s * m + t * n)"
apply (simp add: xzgcd_correct, safe)
apply (rule exI)+
apply (erule xzgcd_linear, auto)
done
lemma zcong_lineq_ex:
"0 < n ==> zgcd (a, n) = 1 ==> \<exists>x. [a * x = 1] (mod n)"
apply (cut_tac m = a and n = n and k = 1 in zgcd_ex_linear, safe)
apply (rule_tac x = s in exI)
apply (rule_tac b = "s * a + t * n" in zcong_trans)
prefer 2
apply simp
apply (unfold zcong_def)
apply (simp (no_asm) add: zmult_commute zdvd_zminus_iff)
done
lemma zcong_lineq_unique:
"0 < n ==>
zgcd (a, n) = 1 ==> \<exists>!x. 0 \<le> x \<and> x < n \<and> [a * x = b] (mod n)"
apply auto
apply (rule_tac [2] zcong_zless_imp_eq)
apply (tactic {* stac (thm "zcong_cancel2" RS sym) 6 *})
apply (rule_tac [8] zcong_trans)
apply (simp_all (no_asm_simp))
prefer 2
apply (simp add: zcong_sym)
apply (cut_tac a = a and n = n in zcong_lineq_ex, auto)
apply (rule_tac x = "x * b mod n" in exI, safe)
apply (simp_all (no_asm_simp))
apply (subst zcong_zmod)
apply (subst zmod_zmult1_eq [symmetric])
apply (subst zcong_zmod [symmetric])
apply (subgoal_tac "[a * x * b = 1 * b] (mod n)")
apply (rule_tac [2] zcong_zmult)
apply (simp_all add: zmult_assoc)
done
end