1 (* ========================================================================= *)
2 (* General "reduction" properties of binary relations, *)
3 (* ========================================================================= *)
5 needs "Library/rstc.ml";;
7 (* ------------------------------------------------------------------------- *)
8 (* Field of a binary relation. *)
9 (* ------------------------------------------------------------------------- *)
11 let FL = new_definition
12 `FL(R) x <=> (?y:A. R x y) \/ (?y. R y x)`;;
14 (* ------------------------------------------------------------------------ *)
15 (* Normality of a term w.r.t. a reduction relation *)
16 (* ------------------------------------------------------------------------ *)
18 let NORMAL = new_definition
19 `NORMAL(R:A->A->bool) x <=> ~(?y. R x y)`;;
21 (* ------------------------------------------------------------------------ *)
22 (* Full Church-Rosser property. *)
24 (* Note that we deviate from most term rewriting literature which call this *)
25 (* the "diamond property" and calls a relation "Church-Rosser" iff its RTC *)
26 (* has the diamond property. But this seems simpler and more natural. *)
27 (* ------------------------------------------------------------------------ *)
29 let CR = new_definition
30 `CR(R:A->A->bool) <=> !x y1 y2. R x y1 /\ R x y2 ==> ?z. R y1 z /\ R y2 z`;;
32 (* ------------------------------------------------------------------------ *)
33 (* Weak Church-Rosser property, i.e. the rejoining may take several steps. *)
34 (* ------------------------------------------------------------------------ *)
36 let WCR = new_definition
37 `WCR(R:A->A->bool) <=>
38 !x y1 y2. R x y1 /\ R x y2 ==> ?z. RTC R y1 z /\ RTC R y2 z`;;
40 (* ------------------------------------------------------------------------ *)
41 (* (Weak) normalization: every term has a normal form. *)
42 (* ------------------------------------------------------------------------ *)
44 let WN = new_definition
45 `WN(R:A->A->bool) <=> !x. ?y. RTC R x y /\ NORMAL(R) y`;;
47 (* ------------------------------------------------------------------------ *)
48 (* Strong normalization: every reduction sequence terminates (Noetherian) *)
49 (* ------------------------------------------------------------------------ *)
51 let SN = new_definition
52 `SN(R:A->A->bool) <=> ~(?seq. !n. R (seq n) (seq (SUC n)))`;;
54 (* ------------------------------------------------------------------------- *)
55 (* Definition of a tree. *)
56 (* ------------------------------------------------------------------------- *)
58 let TREE = new_definition
59 `TREE(R:A->A->bool) <=>
62 !y. y IN FL(R) ==> (y = a) \/ TC R a y /\ ?!x. R x y`;;
64 (* ------------------------------------------------------------------------- *)
65 (* Local finiteness (finitely branching). *)
66 (* ------------------------------------------------------------------------- *)
68 let LF = new_definition
69 `LF(R:A->A->bool) <=> !x. FINITE {y | R x y}`;;
71 (* ------------------------------------------------------------------------- *)
72 (* Wellfoundedness apparatus for SN relations. *)
73 (* ------------------------------------------------------------------------- *)
76 (`!R:A->A->bool. SN(R) <=> WF(INV R)`,
77 REWRITE_TAC[SN; WF_DCHAIN; INV]);;
79 let SN_PRESERVE = prove
80 (`!R:A->A->bool. SN(R) <=> !P. (!x. P x ==> ?y. P y /\ R x y) ==> ~(?x. P x)`,
81 REWRITE_TAC[SN_WF; WF; INV] THEN MESON_TAC[]);;
83 let SN_NOETHERIAN = prove
84 (`!R:A->A->bool. SN(R) <=> !P. (!x. (!y. R x y ==> P y) ==> P x) ==> !x. P x`,
85 REWRITE_TAC[WF_IND; SN_WF; INV]);;
87 (* ------------------------------------------------------------------------ *)
88 (* Normality and weak normalization is preserved by transitive closure. *)
89 (* ------------------------------------------------------------------------ *)
92 (`!R:A->A->bool. NORMAL(TC R) x <=> NORMAL(R) x`,
93 REWRITE_TAC[NORMAL] THEN MESON_TAC[TC_CASES_R; TC_INC]);;
95 let NORMAL_RTC = prove
96 (`!R:A->A->bool. NORMAL(R) x ==> !y. RTC R x y <=> (x = y)`,
97 ONCE_REWRITE_TAC[GSYM NORMAL_TC] THEN
98 REWRITE_TAC[NORMAL; RTC; RC_EXPLICIT] THEN MESON_TAC[]);;
101 (`!R:A->A->bool. WN(TC R) <=> WN R`,
102 REWRITE_TAC[WN; NORMAL_TC; RTC; TC_IDEMP]);;
104 (* ------------------------------------------------------------------------- *)
105 (* Wellfoundedness and strong normalization are too. *)
106 (* ------------------------------------------------------------------------- *)
109 (`!R:A->A->bool. WF(TC R) <=> WF(R)`,
110 GEN_TAC THEN EQ_TAC THENL
111 [MESON_TAC[WF_SUBSET; TC_INC];
112 REWRITE_TAC[WF] THEN DISCH_TAC THEN X_GEN_TAC `P:A->bool` THEN
113 FIRST_X_ASSUM(MP_TAC o SPEC `\y:A. ?z. P z /\ TC(R) z y`) THEN
114 REWRITE_TAC[] THEN MESON_TAC[TC_CASES_L]]);;
116 (******************* Alternative --- intuitionistic --- proof
119 (`!R:A->A->bool. WF(TC R) <=> WF(R)`,
120 GEN_TAC THEN EQ_TAC THENL
121 [MESON_TAC[WF_SUBSET; TC_INC];
122 REWRITE_TAC[WF_IND]] THEN
123 DISCH_TAC THEN GEN_TAC THEN
124 FIRST_ASSUM(MP_TAC o SPEC `\z:A. !u:A. TC(R) u z ==> P(u)`) THEN
125 REWRITE_TAC[] THEN MESON_TAC[TC_CASES_L]);;
127 let WF_TC_EXPLICIT = prove
128 (`!R:A->A->bool. WF(R) ==> WF(TC(R))`,
129 GEN_TAC THEN REWRITE_TAC[WF_IND] THEN DISCH_TAC THEN
130 GEN_TAC THEN DISCH_TAC THEN
131 FIRST_X_ASSUM(MP_TAC o SPEC `\z:A. !u:A. TC(R) u z ==> P(u)`) THEN
132 REWRITE_TAC[] THEN STRIP_TAC THEN X_GEN_TAC `z:A` THEN
133 FIRST_ASSUM MATCH_MP_TAC THEN SPEC_TAC(`z:A`,`z:A`) THEN
134 FIRST_ASSUM MATCH_MP_TAC THEN
135 GEN_TAC THEN GEN_REWRITE_TAC (LAND_CONV o REDEPTH_CONV)
136 [RIGHT_IMP_FORALL_THM; IMP_IMP] THEN
137 DISCH_TAC THEN X_GEN_TAC `u:A` THEN
138 ONCE_REWRITE_TAC[TC_CASES_L] THEN DISCH_THEN(DISJ_CASES_THEN MP_TAC) THENL
140 MATCH_MP_TAC(ASSUME `!x:A. (!y. TC R y x ==> P y) ==> P x`) THEN
141 X_GEN_TAC `v:A` THEN DISCH_TAC THEN FIRST_ASSUM MATCH_MP_TAC THEN
142 EXISTS_TAC `u:A` THEN CONJ_TAC THEN FIRST_ASSUM ACCEPT_TAC;
143 DISCH_THEN(X_CHOOSE_THEN `w:A` STRIP_ASSUME_TAC) THEN
144 FIRST_ASSUM MATCH_MP_TAC THEN EXISTS_TAC `w:A` THEN
145 CONJ_TAC THEN FIRST_ASSUM ACCEPT_TAC]);;
147 ***********************)
150 (`!R:A->A->bool. SN(TC R) <=> SN R`,
151 GEN_TAC THEN REWRITE_TAC[SN_WF; GSYM TC_INV; WF_TC]);;
153 (* ------------------------------------------------------------------------ *)
154 (* Strong normalization implies normalization *)
155 (* ------------------------------------------------------------------------ *)
158 (`!R:A->A->bool. SN(R) ==> WN(R)`,
159 GEN_TAC THEN REWRITE_TAC[SN_WF; WF; WN] THEN DISCH_TAC THEN
160 X_GEN_TAC `a:A` THEN POP_ASSUM(MP_TAC o SPEC `\y:A. RTC R a y`) THEN
161 REWRITE_TAC[INV; NORMAL] THEN MESON_TAC[RTC_REFL; RTC_TRANS_L]);;
163 (* ------------------------------------------------------------------------ *)
164 (* Reflexive closure preserves Church-Rosser property (pretty trivial) *)
165 (* ------------------------------------------------------------------------ *)
168 (`!R:A->A->bool. CR(R) ==> CR(RC R)`,
169 REWRITE_TAC[CR; RC_EXPLICIT] THEN MESON_TAC[]);;
171 (* ------------------------------------------------------------------------ *)
172 (* The strip lemma leads us halfway to the fact that transitive x *)
173 (* closure preserves the Church-Rosser property. It's no harder / \ *)
174 (* to prove it for two separate reduction relations. This then / y2 *)
175 (* allows us to prove the desired theorem simply by using the / / *)
176 (* strip lemma twice with a bit of conjunct-swapping. y1 / *)
178 (* The diagram on the right shows the use of the variables. z *)
179 (* ------------------------------------------------------------------------ *)
181 let STRIP_LEMMA = prove
182 (`!R S. (!x y1 y2. R x y1 /\ S x y2 ==> ?z:A. S y1 z /\ R y2 z) ==>
183 (!x y1 y2. TC R x y1 /\ S x y2 ==> ?z:A. S y1 z /\ TC R y2 z)`,
184 REPEAT GEN_TAC THEN DISCH_TAC THEN
185 REWRITE_TAC[TAUT `a /\ b ==> c <=> a ==> (b ==> c)`] THEN
186 REWRITE_TAC[GSYM RIGHT_IMP_FORALL_THM] THEN
187 MATCH_MP_TAC TC_INDUCT THEN ASM_MESON_TAC[TC_INC; TC_TRANS]);;
189 (* ------------------------------------------------------------------------ *)
190 (* Transitive closure preserves Church-Rosser property. *)
191 (* ------------------------------------------------------------------------ *)
194 (`!R:A->A->bool. CR(R) ==> CR(TC R)`,
195 GEN_TAC THEN REWRITE_TAC[CR] THEN DISCH_TAC THEN
196 MATCH_MP_TAC STRIP_LEMMA THEN REPEAT GEN_TAC THEN
197 ONCE_REWRITE_TAC[CONJ_SYM] THEN
198 RULE_INDUCT_TAC STRIP_LEMMA THEN ASM_REWRITE_TAC[]);;
200 (* ------------------------------------------------------------------------ *)
201 (* Reflexive transitive closure preserves Church-Rosser property. *)
202 (* ------------------------------------------------------------------------ *)
205 (`!R:A->A->bool. CR(R) ==> CR(RTC R)`,
206 REWRITE_TAC[RTC] THEN MESON_TAC[RC_CR; TC_CR]);;
208 (* ------------------------------------------------------------------------ *)
209 (* Equivalent `Church-Rosser` property for the equivalence relation. *)
210 (* ------------------------------------------------------------------------ *)
213 (`!R:A->A->bool. CR(RTC R) <=>
214 !x y. RSTC R x y ==> ?z:A. RTC R x z /\ RTC R y z`,
215 GEN_TAC THEN REWRITE_TAC[CR] THEN EQ_TAC THENL
216 [DISCH_TAC THEN MATCH_MP_TAC RSTC_INDUCT THEN
217 ASM_MESON_TAC[RTC_REFL; RTC_INC; RTC_TRANS];
218 MESON_TAC[RSTC_INC_RTC; RSTC_SYM; RSTC_TRANS]]);;
220 (* ------------------------------------------------------------------------ *)
221 (* Under normalization, Church-Rosser is equivalent to uniqueness of NF *)
222 (* ------------------------------------------------------------------------ *)
225 (`!R:A->A->bool. WN(R) ==>
226 (CR(RTC R) <=> (!x y1 y2. RTC R x y1 /\ NORMAL(R) y1 /\
227 RTC R x y2 /\ NORMAL(R) y2 ==> (y1 = y2)))`,
228 GEN_TAC THEN REWRITE_TAC[CR; WN] THEN DISCH_TAC THEN EQ_TAC THENL
229 [MESON_TAC[NORMAL_RTC]; ASM_MESON_TAC[RTC_TRANS]]);;
231 (* ------------------------------------------------------------------------ *)
232 (* Normalizing and Church-Rosser iff every term has a unique normal form *)
233 (* ------------------------------------------------------------------------ *)
236 (`!R:A->A->bool. WN(R) /\ CR(RTC R) <=> !x. ?!y. RTC R x y /\ NORMAL(R) y`,
237 GEN_TAC THEN ONCE_REWRITE_TAC[EXISTS_UNIQUE_THM] THEN
238 REWRITE_TAC[FORALL_AND_THM; GSYM WN] THEN
239 MATCH_MP_TAC(TAUT `(a ==> (b <=> c)) ==> (a /\ b <=> a /\ c)`) THEN
240 DISCH_THEN(fun th -> REWRITE_TAC[MATCH_MP NORM_CR th]) THEN
241 REWRITE_TAC[CONJ_ASSOC]);;
243 (* ------------------------------------------------------------------------ *)
244 (* Newman's lemma: weak Church-Rosser plus x *)
245 (* strong normalization implies full Church- / \ *)
246 (* Rosser. By the above (and SN ==> WN) it z1 z2 *)
247 (* is sufficient to show normal forms are / | | \ *)
248 (* unique. We use the Noetherian induction / \ / \ *)
249 (* form of SN, so we need only prove that if / z \ *)
250 (* some term has multiple normal forms, so / | \ *)
251 (* does a `successor`. See the diagram on the / | \ *)
252 (* right for the use of variables. y1 w y2 *)
253 (* ------------------------------------------------------------------------ *)
255 let NEWMAN_LEMMA = prove
256 (`!R:A->A->bool. SN(R) /\ WCR(R) ==> CR(RTC R)`,
257 GEN_TAC THEN STRIP_TAC THEN FIRST_ASSUM(MP_TAC o MATCH_MP SN_WN) THEN
258 DISCH_THEN(fun th -> ASSUME_TAC(REWRITE_RULE[WN] th) THEN MP_TAC th) THEN
259 DISCH_THEN(fun th -> REWRITE_TAC[MATCH_MP NORM_CR th]) THEN
260 FIRST_ASSUM(MATCH_MP_TAC o REWRITE_RULE[WF_IND; SN_WF]) THEN
261 REWRITE_TAC[INV] THEN X_GEN_TAC `x:A` THEN REPEAT STRIP_TAC THEN
262 MAP_EVERY UNDISCH_TAC [`RTC R (x:A) y1`; `RTC R (x:A) y2`] THEN
263 ONCE_REWRITE_TAC[RTC_CASES_R] THEN
264 DISCH_THEN(DISJ_CASES_THEN2 ASSUME_TAC (X_CHOOSE_TAC `z2:A`)) THEN
265 DISCH_THEN(DISJ_CASES_THEN2 ASSUME_TAC (X_CHOOSE_TAC `z1:A`)) THENL
266 [ASM_MESON_TAC[];ASM_MESON_TAC[NORMAL];ASM_MESON_TAC[NORMAL]; ALL_TAC] THEN
267 FIRST_ASSUM(MP_TAC o GEN_REWRITE_RULE I [WCR]) THEN
268 ASM_MESON_TAC[RTC_TRANS]);;
270 (* ------------------------------------------------------------------------- *)
271 (* A variant of Koenig's lemma. *)
272 (* ------------------------------------------------------------------------- *)
274 let LF_TC_FINITE = prove
275 (`!R. LF(R) /\ SN(R) ==> !x:A. FINITE {y | TC(R) x y}`,
276 GEN_TAC THEN REWRITE_TAC[LF] THEN STRIP_TAC THEN
277 FIRST_ASSUM(MATCH_MP_TAC o REWRITE_RULE[WF_IND; SN_WF; INV]) THEN
278 GEN_TAC THEN DISCH_TAC THEN SUBGOAL_THEN
279 `{y:A | TC(R) x y} = {y | R x y} UNION
280 (UNIONS { s | ?z. R x z /\ (s = {y | TC(R) z y})})`
282 [REWRITE_TAC[EXTENSION; IN_UNION; IN_UNIONS] THEN
283 REWRITE_TAC[IN_ELIM_THM] THEN REWRITE_TAC[IN] THEN
284 GEN_TAC THEN GEN_REWRITE_TAC LAND_CONV [TC_CASES_R] THEN
285 AP_TERM_TAC THEN EQ_TAC THEN STRIP_TAC THEN ASM_REWRITE_TAC[] THEN
286 ASM_MESON_TAC[]; ALL_TAC] THEN
287 ASM_REWRITE_TAC[FINITE_UNION; FINITE_UNIONS] THEN CONJ_TAC THENL
288 [MP_TAC(ISPECL [`\z:A. {y | TC R z y}`; `{z | (R:A->A->bool) x z}`]
289 FINITE_IMAGE_EXPAND) THEN
290 ASM_REWRITE_TAC[] THEN REWRITE_TAC[IN; IN_ELIM_THM];
291 GEN_TAC THEN GEN_REWRITE_TAC LAND_CONV [IN_ELIM_THM] THEN
292 REWRITE_TAC[] THEN STRIP_TAC THEN ASM_REWRITE_TAC[] THEN
293 FIRST_ASSUM MATCH_MP_TAC THEN FIRST_ASSUM ACCEPT_TAC]);;
295 let SN_NOLOOP = prove
296 (`!R:A->A->bool. SN(R) ==> !z. ~(TC(R) z z)`,
297 GEN_TAC THEN ONCE_REWRITE_TAC[GSYM SN_TC] THEN
298 SPEC_TAC(`TC(R:A->A->bool)`,`R:A->A->bool`) THEN
299 GEN_TAC THEN REWRITE_TAC[SN_WF; INV; WF] THEN
300 DISCH_THEN(fun th -> GEN_TAC THEN MP_TAC th) THEN
301 DISCH_THEN(MP_TAC o SPEC `\x:A. x = z`) THEN
302 REWRITE_TAC[] THEN MESON_TAC[]);;
304 let RELPOW_RTC = prove
305 (`!R:A->A->bool. !n x y. RELPOW n R x y ==> RTC(R) x y`,
306 GEN_TAC THEN INDUCT_TAC THEN REWRITE_TAC[RELPOW] THEN
307 ASM_MESON_TAC[RTC_REFL; RTC_TRANS_L]);;
309 let RTC_TC_LEMMA = prove
310 (`!R x:A. {y:A | RTC(R) x y} = x INSERT {y:A | TC(R) x y}`,
311 REWRITE_TAC[EXTENSION; IN_ELIM_THM; IN_INSERT] THEN
312 REWRITE_TAC[RTC; RC_EXPLICIT; DISJ_ACI; EQ_SYM_EQ]);;
314 let HAS_SIZE_SUBSET = prove
315 (`!s:A->bool t m n. s HAS_SIZE m /\ t HAS_SIZE n /\ s SUBSET t ==> m <= n`,
316 REWRITE_TAC[HAS_SIZE] THEN MESON_TAC[CARD_SUBSET]);;
318 let FC_FINITE_BOUND_LEMMA = prove
319 (`!R. (!z. ~(TC R z z))
320 ==> !n. {y:A | RTC(R) x y} HAS_SIZE n
321 ==> !m y. RELPOW m R x y ==> m <= n`,
322 REPEAT STRIP_TAC THEN
323 FIRST_ASSUM(X_CHOOSE_THEN `f:num->A` STRIP_ASSUME_TAC o
324 REWRITE_RULE[RELPOW_SEQUENCE]) THEN
325 SUBGOAL_THEN `!i. i <= m ==> RELPOW i R (x:A) (f i)` ASSUME_TAC THENL
326 [INDUCT_TAC THEN ASM_REWRITE_TAC[RELPOW] THEN
327 REWRITE_TAC[LE_SUC_LT] THEN ASM_MESON_TAC[LT_IMP_LE]; ALL_TAC] THEN
328 SUBGOAL_THEN `{z:A | ?i:num. i < m /\ (z = f i)} SUBSET {y | RTC R x y}`
330 [REWRITE_TAC[SUBSET; IN_ELIM_THM] THEN ASM_MESON_TAC[RELPOW_RTC; LT_IMP_LE];
332 SUBGOAL_THEN `!p. p <= m ==> {z:A | ?i. i < p /\ (z = f i)} HAS_SIZE p`
333 (fun th -> ASSUME_TAC(MATCH_MP th (SPEC `m:num` LE_REFL))) THENL
335 MATCH_MP_TAC HAS_SIZE_SUBSET THEN
336 EXISTS_TAC `{z:A | ?i. i < m /\ (z = f i)}` THEN
337 EXISTS_TAC `{y:A | RTC(R) x y}` THEN ASM_REWRITE_TAC[]] THEN
338 INDUCT_TAC THEN DISCH_TAC THENL
339 [REWRITE_TAC[HAS_SIZE_0; EXTENSION; NOT_IN_EMPTY; IN_ELIM_THM; LT];
341 SUBGOAL_THEN `{z:A | ?i. i < SUC p /\ (z = f i)} =
342 f(p) INSERT {z | ?i. i < p /\ (z = f i)}`
344 [REWRITE_TAC[EXTENSION; IN_INSERT; IN_ELIM_THM] THEN
345 REWRITE_TAC[LT] THEN MESON_TAC[]; ALL_TAC] THEN
346 REWRITE_TAC[HAS_SIZE; CARD_CLAUSES; SUC_INJ] THEN
347 SUBGOAL_THEN `{z:A | ?i. i < p /\ (z = f i)} HAS_SIZE p` MP_TAC THENL
348 [FIRST_ASSUM MATCH_MP_TAC THEN UNDISCH_TAC `SUC p <= m` THEN ARITH_TAC;
350 REWRITE_TAC[HAS_SIZE] THEN STRIP_TAC THEN
351 FIRST_ASSUM(fun th -> REWRITE_TAC[MATCH_MP (CONJUNCT2 CARD_CLAUSES) th]) THEN
352 COND_CASES_TAC THEN ASM_REWRITE_TAC[FINITE_INSERT] THEN
353 UNDISCH_TAC `f p IN {z:A | ?i:num. i < p /\ (z = f i)}` THEN
354 CONV_TAC CONTRAPOS_CONV THEN DISCH_TAC THEN
355 REWRITE_TAC[IN_ELIM_THM; NOT_EXISTS_THM] THEN
356 X_GEN_TAC `q:num` THEN
357 DISCH_THEN(CONJUNCTS_THEN2 ASSUME_TAC MP_TAC) THEN
358 SUBGOAL_THEN `TC(R) ((f:num->A) q) (f p)` (fun th -> ASM_MESON_TAC[th]) THEN
359 UNDISCH_TAC `SUC p <= m` THEN UNDISCH_TAC `q < p` THEN
360 REWRITE_TAC[LT_EXISTS] THEN
361 DISCH_THEN(X_CHOOSE_THEN `d:num` SUBST1_TAC) THEN
362 SPEC_TAC(`d:num`,`d:num`) THEN INDUCT_TAC THENL
363 [REWRITE_TAC[ADD_CLAUSES] THEN DISCH_TAC THEN
364 MATCH_MP_TAC TC_INC THEN FIRST_ASSUM MATCH_MP_TAC THEN
365 UNDISCH_TAC `SUC (SUC q) <= m` THEN ARITH_TAC;
366 DISCH_TAC THEN MATCH_MP_TAC TC_TRANS_L THEN
367 EXISTS_TAC `(f:num->A)(q + SUC d)` THEN CONJ_TAC THENL
368 [ALL_TAC; REWRITE_TAC[ADD_CLAUSES]] THEN
369 FIRST_ASSUM MATCH_MP_TAC THEN
370 UNDISCH_TAC `SUC (q + SUC (SUC d)) <= m` THEN ARITH_TAC]);;
372 let FC_FINITE_BOUND = prove
373 (`!R (x:A). FINITE {y | RTC(R) x y} /\
375 ==> ?N. !n y. RELPOW n R x y ==> n <= N`,
376 REPEAT GEN_TAC THEN DISCH_THEN(CONJUNCTS_THEN2 MP_TAC ASSUME_TAC) THEN
377 DISCH_TAC THEN EXISTS_TAC `CARD {y:A | RTC(R) x y}` THEN
378 FIRST_ASSUM(MATCH_MP_TAC o MATCH_MP FC_FINITE_BOUND_LEMMA) THEN
379 ASM_REWRITE_TAC[HAS_SIZE]);;
382 (`!R. (!x:A. ?N. !n y. RELPOW n R x y ==> n <= N) ==> SN(R)`,
383 GEN_TAC THEN DISCH_TAC THEN REWRITE_TAC[SN_WF; WF_DCHAIN; INV] THEN
384 DISCH_THEN(X_CHOOSE_TAC `f:num->A`) THEN
385 FIRST_X_ASSUM(MP_TAC o SPEC `(f:num->A) 0`) THEN
386 DISCH_THEN(X_CHOOSE_THEN `N:num`
387 (MP_TAC o SPECL [`SUC N`; `f(SUC N):A`])) THEN
388 REWRITE_TAC[GSYM NOT_LT; LT] THEN
389 SUBGOAL_THEN `!n. RELPOW n R (f 0 :A) (f n)` (fun th -> REWRITE_TAC[th]) THEN
390 INDUCT_TAC THEN ASM_REWRITE_TAC[RELPOW] THEN ASM_MESON_TAC[]);;
392 let LF_SN_BOUND = prove
393 (`!R. LF(R) ==> (SN(R) <=> !x:A. ?N. !n y. RELPOW n R x y ==> n <= N)`,
394 GEN_TAC THEN DISCH_TAC THEN EQ_TAC THEN REWRITE_TAC[BOUND_SN] THEN
395 DISCH_TAC THEN GEN_TAC THEN MATCH_MP_TAC FC_FINITE_BOUND THEN CONJ_TAC THENL
396 [SPEC_TAC(`x:A`,`x:A`) THEN REWRITE_TAC[RTC_TC_LEMMA; FINITE_INSERT] THEN
397 MATCH_MP_TAC LF_TC_FINITE THEN ASM_REWRITE_TAC[];
398 MATCH_MP_TAC SN_NOLOOP THEN ASM_REWRITE_TAC[]]);;
400 (* ------------------------------------------------------------------------- *)
401 (* Koenig's lemma. *)
402 (* ------------------------------------------------------------------------- *)
405 (`!R. TREE(R) ==> ?a:A. FL(R) = {y | RTC(R) a y}`,
406 GEN_TAC THEN REWRITE_TAC[TREE] THEN
407 DISCH_THEN(CONJUNCTS_THEN2 ASSUME_TAC
408 (X_CHOOSE_THEN `a:A` STRIP_ASSUME_TAC)) THEN
409 EXISTS_TAC `a:A` THEN REWRITE_TAC[EXTENSION; IN_ELIM_THM] THEN
410 X_GEN_TAC `x:A` THEN EQ_TAC THENL
411 [DISCH_THEN(ANTE_RES_THEN MP_TAC) THEN REWRITE_TAC[RTC; RC_EXPLICIT] THEN
412 MESON_TAC[]; ONCE_REWRITE_TAC[RTC_CASES_L] THEN ASM_MESON_TAC[IN; FL]]);;
414 let KOENIG_LEMMA = prove
415 (`!R:A->A->bool. TREE(R) /\ LF(R) /\ SN(R) ==> FINITE (FL R)`,
416 GEN_TAC THEN DISCH_THEN(CONJUNCTS_THEN2 MP_TAC STRIP_ASSUME_TAC) THEN
417 DISCH_THEN(X_CHOOSE_THEN `a:A` SUBST1_TAC o MATCH_MP TREE_FL) THEN
418 REWRITE_TAC[RTC_TC_LEMMA; FINITE_INSERT] THEN
419 SPEC_TAC(`a:A`,`a:A`) THEN MATCH_MP_TAC LF_TC_FINITE THEN
422 (* ------------------------------------------------------------------------- *)
423 (* Rephrasing in terms of joinability. *)
424 (* ------------------------------------------------------------------------- *)
426 let JOINABLE = new_definition
427 `JOINABLE R s t <=> ?u. RTC R s u /\ RTC R t u`;;
429 let JOINABLE_REFL = prove
430 (`!R t. JOINABLE R t t`,
431 REWRITE_TAC[JOINABLE] THEN MESON_TAC[RTC_CASES]);;
433 let JOINABLE_SYM = prove
434 (`!R s t. JOINABLE R s t <=> JOINABLE R t s`,
435 REWRITE_TAC[JOINABLE] THEN MESON_TAC[]);;
437 let JOINABLE_TRANS_R = prove
438 (`!R s t u. R s t /\ JOINABLE R t u ==> JOINABLE R s u`,
439 REWRITE_TAC[JOINABLE] THEN MESON_TAC[RTC_CASES_R]);;
441 let CR_RSTC_JOINABLE = prove
442 (`!R. CR(RTC R) ==> !x:A y. RSTC(R) x y <=> JOINABLE(R) x y`,
443 GEN_TAC THEN REWRITE_TAC[STC_CR; JOINABLE] THEN
444 ASM_MESON_TAC[RSTC_TRANS; RSTC_SYM; RSTC_INC_RTC]);;
446 (* ------------------------------------------------------------------------- *)
447 (* CR is equivalent to transitivity of joinability. *)
448 (* ------------------------------------------------------------------------- *)
450 let JOINABLE_TRANS = prove
452 !x y z. JOINABLE(R) x y /\ JOINABLE(R) y z ==> JOINABLE(R) x z`,
453 REWRITE_TAC[CR; JOINABLE] THEN MESON_TAC[RTC_REFL; RTC_TRANS; RTC_SYM]);;