1 (* Title: HOL/Tools/meson.ML
3 Author: Lawrence C Paulson, Cambridge University Computer Laboratory
4 Copyright 1992 University of Cambridge
6 The MESON resolution proof procedure for HOL.
8 When making clauses, avoids using the rewriter -- instead uses RS recursively
10 NEED TO SORT LITERALS BY # OF VARS, USING ==>I/E. ELIMINATES NEED FOR
11 FUNCTION nodups -- if done to goal clauses too!
14 signature BASIC_MESON =
16 val size_of_subgoals : thm -> int
17 val make_cnf : thm list -> thm -> thm list
18 val make_nnf : thm -> thm
19 val make_nnf1 : thm -> thm
20 val skolemize : thm -> thm
21 val make_clauses : thm list -> thm list
22 val make_horns : thm list -> thm list
23 val best_prolog_tac : (thm -> int) -> thm list -> tactic
24 val depth_prolog_tac : thm list -> tactic
25 val gocls : thm list -> thm list
26 val skolemize_prems_tac : thm list -> int -> tactic
27 val MESON : (thm list -> tactic) -> int -> tactic
28 val best_meson_tac : (thm -> int) -> int -> tactic
29 val safe_best_meson_tac : int -> tactic
30 val depth_meson_tac : int -> tactic
31 val prolog_step_tac' : thm list -> int -> tactic
32 val iter_deepen_prolog_tac : thm list -> tactic
33 val iter_deepen_meson_tac : thm list -> int -> tactic
34 val meson_tac : int -> tactic
35 val negate_head : thm -> thm
36 val select_literal : int -> thm -> thm
37 val skolemize_tac : int -> tactic
38 val make_clauses_tac : int -> tactic
39 val check_is_fol_term : term -> term
46 val not_conjD = thm "meson_not_conjD";
47 val not_disjD = thm "meson_not_disjD";
48 val not_notD = thm "meson_not_notD";
49 val not_allD = thm "meson_not_allD";
50 val not_exD = thm "meson_not_exD";
51 val imp_to_disjD = thm "meson_imp_to_disjD";
52 val not_impD = thm "meson_not_impD";
53 val iff_to_disjD = thm "meson_iff_to_disjD";
54 val not_iffD = thm "meson_not_iffD";
55 val conj_exD1 = thm "meson_conj_exD1";
56 val conj_exD2 = thm "meson_conj_exD2";
57 val disj_exD = thm "meson_disj_exD";
58 val disj_exD1 = thm "meson_disj_exD1";
59 val disj_exD2 = thm "meson_disj_exD2";
60 val disj_assoc = thm "meson_disj_assoc";
61 val disj_comm = thm "meson_disj_comm";
62 val disj_FalseD1 = thm "meson_disj_FalseD1";
63 val disj_FalseD2 = thm "meson_disj_FalseD2";
65 val depth_limit = ref 2000;
67 (**** Operators for forward proof ****)
69 (*Like RS, but raises Option if there are no unifiers and allows multiple unifiers.*)
70 fun resolve1 (tha,thb) = Seq.hd (biresolution false [(false,tha)] 1 thb);
72 (*raises exception if no rules apply -- unlike RL*)
73 fun tryres (th, rls) =
74 let fun tryall [] = raise THM("tryres", 0, th::rls)
75 | tryall (rl::rls) = (resolve1(th,rl) handle Option.Option => tryall rls)
78 (*Permits forward proof from rules that discharge assumptions*)
79 fun forward_res nf st =
80 case Seq.pull (ALLGOALS (METAHYPS (fn [prem] => rtac (nf prem) 1)) st)
82 | NONE => raise THM("forward_res", 0, [st]);
85 (*Are any of the constants in "bs" present in the term?*)
87 let fun has (Const(a,_)) = member (op =) bs a
88 | has (Const ("Hilbert_Choice.Eps",_) $ _) = false
89 (*ignore constants within @-terms*)
90 | has (f$u) = has f orelse has u
91 | has (Abs(_,_,t)) = has t
96 (**** Clause handling ****)
98 fun literals (Const("Trueprop",_) $ P) = literals P
99 | literals (Const("op |",_) $ P $ Q) = literals P @ literals Q
100 | literals (Const("Not",_) $ P) = [(false,P)]
101 | literals P = [(true,P)];
103 (*number of literals in a term*)
104 val nliterals = length o literals;
107 (*** Tautology Checking ***)
109 fun signed_lits_aux (Const ("op |", _) $ P $ Q) (poslits, neglits) =
110 signed_lits_aux Q (signed_lits_aux P (poslits, neglits))
111 | signed_lits_aux (Const("Not",_) $ P) (poslits, neglits) = (poslits, P::neglits)
112 | signed_lits_aux P (poslits, neglits) = (P::poslits, neglits);
114 fun signed_lits th = signed_lits_aux (HOLogic.dest_Trueprop (concl_of th)) ([],[]);
116 (*Literals like X=X are tautologous*)
117 fun taut_poslit (Const("op =",_) $ t $ u) = t aconv u
118 | taut_poslit (Const("True",_)) = true
119 | taut_poslit _ = false;
122 let val (poslits,neglits) = signed_lits th
123 in exists taut_poslit poslits
125 exists (fn t => mem_term (t, neglits)) (HOLogic.false_const :: poslits)
127 handle TERM _ => false; (*probably dest_Trueprop on a weird theorem*)
130 (*** To remove trivial negated equality literals from clauses ***)
132 (*They are typically functional reflexivity axioms and are the converses of
133 injectivity equivalences*)
135 val not_refl_disj_D = thm"meson_not_refl_disj_D";
137 fun refl_clause_aux 0 th = th
138 | refl_clause_aux n th =
139 case HOLogic.dest_Trueprop (concl_of th) of
140 (Const ("op |", _) $ (Const ("op |", _) $ _ $ _) $ _) =>
141 refl_clause_aux n (th RS disj_assoc) (*isolate an atom as first disjunct*)
142 | (Const ("op |", _) $ (Const("Not",_) $ (Const("op =",_) $ t $ u)) $ _) =>
143 if is_Var t orelse is_Var u then (*Var inequation: delete or ignore*)
144 (refl_clause_aux (n-1) (th RS not_refl_disj_D) (*delete*)
145 handle THM _ => refl_clause_aux (n-1) (th RS disj_comm)) (*ignore*)
146 else refl_clause_aux (n-1) (th RS disj_comm) (*not between Vars: ignore*)
147 | (Const ("op |", _) $ _ $ _) => refl_clause_aux n (th RS disj_comm)
148 | _ => (*not a disjunction*) th;
150 fun notequal_lits_count (Const ("op |", _) $ P $ Q) =
151 notequal_lits_count P + notequal_lits_count Q
152 | notequal_lits_count (Const("Not",_) $ (Const("op =",_) $ _ $ _)) = 1
153 | notequal_lits_count _ = 0;
155 (*Simplify a clause by applying reflexivity to its negated equality literals*)
157 let val neqs = notequal_lits_count (HOLogic.dest_Trueprop (concl_of th))
158 in zero_var_indexes (refl_clause_aux neqs th) end
159 handle TERM _ => th; (*probably dest_Trueprop on a weird theorem*)
162 (*** The basic CNF transformation ***)
164 (*Estimate the number of clauses in order to detect infeasible theorems*)
165 fun nclauses (Const("Trueprop",_) $ t) = nclauses t
166 | nclauses (Const("op &",_) $ t $ u) = nclauses t + nclauses u
167 | nclauses (Const("Ex", _) $ Abs (_,_,t)) = nclauses t
168 | nclauses (Const("All",_) $ Abs (_,_,t)) = nclauses t
169 | nclauses (Const("op |",_) $ t $ u) = nclauses t * nclauses u
170 | nclauses _ = 1; (* literal *)
172 (*Replaces universally quantified variables by FREE variables -- because
173 assumptions may not contain scheme variables. Later, call "generalize". *)
175 let val newname = gensym "A_"
176 val spec' = read_instantiate [("x", newname)] spec
179 (*Used with METAHYPS below. There is one assumption, which gets bound to prem
180 and then normalized via function nf. The normal form is given to resolve_tac,
181 presumably to instantiate a Boolean variable.*)
182 fun resop nf [prem] = resolve_tac (nf prem) 1;
185 exists_Const (fn (c,_) => c mem_string ["==", "==>", "all", "prop"]);
187 (*Conjunctive normal form, adding clauses from th in front of ths (for foldr).
188 Strips universal quantifiers and breaks up conjunctions.
189 Eliminates existential quantifiers using skoths: Skolemization theorems.*)
190 fun cnf skoths (th,ths) =
191 let fun cnf_aux (th,ths) =
192 if has_meta_conn (prop_of th) then ths (*meta-level: ignore*)
193 else if not (has_consts ["All","Ex","op &"] (prop_of th))
194 then th::ths (*no work to do, terminate*)
195 else case head_of (HOLogic.dest_Trueprop (concl_of th)) of
196 Const ("op &", _) => (*conjunction*)
197 cnf_aux (th RS conjunct1,
198 cnf_aux (th RS conjunct2, ths))
199 | Const ("All", _) => (*universal quantifier*)
200 cnf_aux (freeze_spec th, ths)
202 (*existential quantifier: Insert Skolem functions*)
203 cnf_aux (tryres (th,skoths), ths)
204 | Const ("op |", _) => (*disjunction*)
206 (METAHYPS (resop cnf_nil) 1) THEN
207 (fn st' => st' |> METAHYPS (resop cnf_nil) 1)
208 in Seq.list_of (tac (th RS disj_forward)) @ ths end
209 | _ => (*no work to do*) th::ths
210 and cnf_nil th = cnf_aux (th,[])
212 if nclauses (concl_of th) > 20
213 then (Output.debug ("cnf is ignoring: " ^ string_of_thm th); ths)
214 else cnf_aux (th,ths)
217 (*Convert all suitable free variables to schematic variables,
218 but don't discharge assumptions.*)
219 fun generalize th = Thm.varifyT (forall_elim_vars 0 (forall_intr_frees th));
221 fun make_cnf skoths th =
222 filter (not o is_taut)
223 (map (refl_clause o generalize) (cnf skoths (th, [])));
226 (**** Removal of duplicate literals ****)
228 (*Forward proof, passing extra assumptions as theorems to the tactic*)
229 fun forward_res2 nf hyps st =
232 (METAHYPS (fn major::minors => rtac (nf (minors@hyps) major) 1) 1)
235 | NONE => raise THM("forward_res2", 0, [st]);
237 (*Remove duplicates in P|Q by assuming ~P in Q
238 rls (initially []) accumulates assumptions of the form P==>False*)
239 fun nodups_aux rls th = nodups_aux rls (th RS disj_assoc)
240 handle THM _ => tryres(th,rls)
241 handle THM _ => tryres(forward_res2 nodups_aux rls (th RS disj_forward2),
242 [disj_FalseD1, disj_FalseD2, asm_rl])
245 (*Remove duplicate literals, if there are any*)
247 if null(findrep(literals(prop_of th))) then th
248 else nodups_aux [] th;
251 (**** Generation of contrapositives ****)
253 (*Associate disjuctions to right -- make leftmost disjunct a LITERAL*)
254 fun assoc_right th = assoc_right (th RS disj_assoc)
257 (*Must check for negative literal first!*)
258 val clause_rules = [disj_assoc, make_neg_rule, make_pos_rule];
260 (*For ordinary resolution. *)
261 val resolution_clause_rules = [disj_assoc, make_neg_rule', make_pos_rule'];
263 (*Create a goal or support clause, conclusing False*)
264 fun make_goal th = (*Must check for negative literal first!*)
265 make_goal (tryres(th, clause_rules))
266 handle THM _ => tryres(th, [make_neg_goal, make_pos_goal]);
268 (*Sort clauses by number of literals*)
269 fun fewerlits(th1,th2) = nliterals(prop_of th1) < nliterals(prop_of th2);
271 fun sort_clauses ths = sort (make_ord fewerlits) ths;
273 (*True if the given type contains bool anywhere*)
274 fun has_bool (Type("bool",_)) = true
275 | has_bool (Type(_, Ts)) = exists has_bool Ts
276 | has_bool _ = false;
278 (*Is the string the name of a connective? It doesn't matter if this list is
279 incomplete, since when actually called, the only connectives likely to
280 remain are & | Not.*)
281 val is_conn = member (op =)
282 ["Trueprop", "HOL.tag", "op &", "op |", "op -->", "op =", "Not",
283 "All", "Ex", "Ball", "Bex"];
285 (*True if the term contains a function where the type of any argument contains
287 val has_bool_arg_const =
289 (fn (c,T) => not(is_conn c) andalso exists (has_bool) (binder_types T));
291 (*Raises an exception if any Vars in the theorem mention type bool; they
292 could cause make_horn to loop! Also rejects functions whose arguments are
293 Booleans or other functions.*)
295 not (exists (has_bool o fastype_of) (term_vars t) orelse
296 not (Term.is_first_order ["all","All","Ex"] t) orelse
297 has_bool_arg_const t orelse
300 (*FIXME: replace this by the boolean-valued version above!*)
301 fun check_is_fol_term t =
302 if is_fol_term t then t else raise TERM("check_is_fol_term",[t]);
304 fun check_is_fol th = (check_is_fol_term (prop_of th); th);
307 (*Create a meta-level Horn clause*)
308 fun make_horn crules th = make_horn crules (tryres(th,crules))
311 (*Generate Horn clauses for all contrapositives of a clause. The input, th,
312 is a HOL disjunction.*)
313 fun add_contras crules (th,hcs) =
314 let fun rots (0,th) = hcs
315 | rots (k,th) = zero_var_indexes (make_horn crules th) ::
316 rots(k-1, assoc_right (th RS disj_comm))
317 in case nliterals(prop_of th) of
319 | n => rots(n, assoc_right th)
322 (*Use "theorem naming" to label the clauses*)
323 fun name_thms label =
324 let fun name1 (th, (k,ths)) =
325 (k-1, Thm.name_thm (label ^ string_of_int k, th) :: ths)
327 in fn ths => #2 (foldr name1 (length ths, []) ths) end;
329 (*Is the given disjunction an all-negative support clause?*)
330 fun is_negative th = forall (not o #1) (literals (prop_of th));
332 val neg_clauses = List.filter is_negative;
335 (***** MESON PROOF PROCEDURE *****)
337 fun rhyps (Const("==>",_) $ (Const("Trueprop",_) $ A) $ phi,
338 As) = rhyps(phi, A::As)
339 | rhyps (_, As) = As;
341 (** Detecting repeated assumptions in a subgoal **)
343 (*The stringtree detects repeated assumptions.*)
344 fun ins_term (net,t) = Net.insert_term (op aconv) (t,t) net;
346 (*detects repetitions in a list of terms*)
347 fun has_reps [] = false
348 | has_reps [_] = false
349 | has_reps [t,u] = (t aconv u)
350 | has_reps ts = (Library.foldl ins_term (Net.empty, ts); false)
351 handle Net.INSERT => true;
353 (*Like TRYALL eq_assume_tac, but avoids expensive THEN calls*)
354 fun TRYING_eq_assume_tac 0 st = Seq.single st
355 | TRYING_eq_assume_tac i st =
356 TRYING_eq_assume_tac (i-1) (eq_assumption i st)
357 handle THM _ => TRYING_eq_assume_tac (i-1) st;
359 fun TRYALL_eq_assume_tac st = TRYING_eq_assume_tac (nprems_of st) st;
361 (*Loop checking: FAIL if trying to prove the same thing twice
362 -- if *ANY* subgoal has repeated literals*)
364 if exists (fn prem => has_reps (rhyps(prem,[]))) (prems_of st)
365 then Seq.empty else Seq.single st;
368 (* net_resolve_tac actually made it slower... *)
369 fun prolog_step_tac horns i =
370 (assume_tac i APPEND resolve_tac horns i) THEN check_tac THEN
371 TRYALL_eq_assume_tac;
373 (*Sums the sizes of the subgoals, ignoring hypotheses (ancestors)*)
374 fun addconcl(prem,sz) = size_of_term(Logic.strip_assums_concl prem) + sz
376 fun size_of_subgoals st = foldr addconcl 0 (prems_of st);
379 (*Negation Normal Form*)
380 val nnf_rls = [imp_to_disjD, iff_to_disjD, not_conjD, not_disjD,
381 not_impD, not_iffD, not_allD, not_exD, not_notD];
383 fun make_nnf1 th = make_nnf1 (tryres(th, nnf_rls))
385 forward_res make_nnf1
386 (tryres(th, [conj_forward,disj_forward,all_forward,ex_forward]))
389 (*The simplification removes defined quantifiers and occurrences of True and False,
390 as well as tags applied to True and False. nnf_ss also includes the one-point simprocs,
391 which are needed to avoid the various one-point theorems from generating junk clauses.*)
392 val tag_True = thm "tag_True";
393 val tag_False = thm "tag_False";
395 [simp_implies_def, Ex1_def, Ball_def, Bex_def, tag_True, tag_False, if_True,
396 if_False, if_cancel, if_eq_cancel, cases_simp];
397 val nnf_extra_simps =
398 thms"split_ifs" @ ex_simps @ all_simps @ simp_thms;
401 HOL_basic_ss addsimps nnf_extra_simps
402 addsimprocs [defALL_regroup,defEX_regroup,neq_simproc,let_simproc];
404 fun make_nnf th = th |> rewrite_rule (map safe_mk_meta_eq nnf_simps)
405 |> simplify nnf_ss (*But this doesn't simplify premises...*)
408 (*Pull existential quantifiers to front. This accomplishes Skolemization for
409 clauses that arise from a subgoal.*)
411 if not (has_consts ["Ex"] (prop_of th)) then th
412 else (skolemize (tryres(th, [choice, conj_exD1, conj_exD2,
413 disj_exD, disj_exD1, disj_exD2])))
415 skolemize (forward_res skolemize
416 (tryres (th, [conj_forward, disj_forward, all_forward])))
417 handle THM _ => forward_res skolemize (th RS ex_forward);
420 (*Make clauses from a list of theorems, previously Skolemized and put into nnf.
421 The resulting clauses are HOL disjunctions.*)
422 fun make_clauses ths =
423 (sort_clauses (map (generalize o nodups) (foldr (cnf[]) [] ths)));
426 (*Convert a list of clauses (disjunctions) to Horn clauses (contrapositives)*)
429 (distinct Drule.eq_thm_prop (foldr (add_contras clause_rules) [] ths));
431 (*Could simply use nprems_of, which would count remaining subgoals -- no
432 discrimination as to their size! With BEST_FIRST, fails for problem 41.*)
434 fun best_prolog_tac sizef horns =
435 BEST_FIRST (has_fewer_prems 1, sizef) (prolog_step_tac horns 1);
437 fun depth_prolog_tac horns =
438 DEPTH_FIRST (has_fewer_prems 1) (prolog_step_tac horns 1);
440 (*Return all negative clauses, as possible goal clauses*)
441 fun gocls cls = name_thms "Goal#" (map make_goal (neg_clauses cls));
443 fun skolemize_prems_tac prems =
444 cut_facts_tac (map (skolemize o make_nnf) prems) THEN'
447 (*Expand all definitions (presumably of Skolem functions) in a proof state.*)
448 fun expand_defs_tac st =
449 let val defs = filter (can dest_equals) (#hyps (crep_thm st))
450 in LocalDefs.def_export false defs st end;
452 (*Basis of all meson-tactics. Supplies cltac with clauses: HOL disjunctions*)
453 fun MESON cltac i st =
455 (EVERY [rtac ccontr 1,
457 EVERY1 [skolemize_prems_tac negs,
458 METAHYPS (cltac o make_clauses)]) 1,
459 expand_defs_tac]) i st
460 handle TERM _ => no_tac st; (*probably from check_is_fol*)
462 (** Best-first search versions **)
464 (*ths is a list of additional clauses (HOL disjunctions) to use.*)
465 fun best_meson_tac sizef =
467 THEN_BEST_FIRST (resolve_tac (gocls cls) 1)
468 (has_fewer_prems 1, sizef)
469 (prolog_step_tac (make_horns cls) 1));
471 (*First, breaks the goal into independent units*)
472 val safe_best_meson_tac =
473 SELECT_GOAL (TRY Safe_tac THEN
474 TRYALL (best_meson_tac size_of_subgoals));
476 (** Depth-first search version **)
478 val depth_meson_tac =
479 MESON (fn cls => EVERY [resolve_tac (gocls cls) 1,
480 depth_prolog_tac (make_horns cls)]);
483 (** Iterative deepening version **)
485 (*This version does only one inference per call;
486 having only one eq_assume_tac speeds it up!*)
487 fun prolog_step_tac' horns =
488 let val (horn0s, hornps) = (*0 subgoals vs 1 or more*)
489 take_prefix Thm.no_prems horns
490 val nrtac = net_resolve_tac horns
491 in fn i => eq_assume_tac i ORELSE
492 match_tac horn0s i ORELSE (*no backtracking if unit MATCHES*)
493 ((assume_tac i APPEND nrtac i) THEN check_tac)
496 fun iter_deepen_prolog_tac horns =
497 ITER_DEEPEN (has_fewer_prems 1) (prolog_step_tac' horns);
499 fun iter_deepen_meson_tac ths =
501 case (gocls (cls@ths)) of
502 [] => no_tac (*no goal clauses*)
504 (THEN_ITER_DEEPEN (resolve_tac goes 1)
506 (prolog_step_tac' (make_horns (cls@ths)))));
508 fun meson_claset_tac ths cs =
509 SELECT_GOAL (TRY (safe_tac cs) THEN TRYALL (iter_deepen_meson_tac ths));
511 val meson_tac = CLASET' (meson_claset_tac []);
514 (**** Code to support ordinary resolution, rather than Model Elimination ****)
516 (*Convert a list of clauses (disjunctions) to meta-level clauses (==>),
517 with no contrapositives, for ordinary resolution.*)
519 (*Rules to convert the head literal into a negated assumption. If the head
520 literal is already negated, then using notEfalse instead of notEfalse'
521 prevents a double negation.*)
522 val notEfalse = read_instantiate [("R","False")] notE;
523 val notEfalse' = rotate_prems 1 notEfalse;
525 fun negated_asm_of_head th =
526 th RS notEfalse handle THM _ => th RS notEfalse';
528 (*Converting one clause*)
529 fun make_meta_clause th =
530 negated_asm_of_head (make_horn resolution_clause_rules (check_is_fol th));
532 fun make_meta_clauses ths =
534 (distinct Drule.eq_thm_prop (map make_meta_clause ths));
536 (*Permute a rule's premises to move the i-th premise to the last position.*)
538 let val n = nprems_of th
539 in if 1 <= i andalso i <= n
540 then Thm.permute_prems (i-1) 1 th
541 else raise THM("select_literal", i, [th])
544 (*Maps a rule that ends "... ==> P ==> False" to "... ==> ~P" while suppressing
546 val negate_head = rewrite_rule [atomize_not, not_not RS eq_reflection];
548 (*Maps the clause [P1,...Pn]==>False to [P1,...,P(i-1),P(i+1),...Pn] ==> ~P*)
549 fun select_literal i cl = negate_head (make_last i cl);
552 (*Top-level Skolemization. Allows part of the conversion to clauses to be
553 expressed as a tactic (or Isar method). Each assumption of the selected
554 goal is converted to NNF and then its existential quantifiers are pulled
555 to the front. Finally, all existential quantifiers are eliminated,
556 leaving !!-quantified variables. Perhaps Safe_tac should follow, but it
557 might generate many subgoals.*)
559 fun skolemize_tac i st =
560 let val ts = Logic.strip_assums_hyp (List.nth (prems_of st, i-1))
563 (fn hyps => (cut_facts_tac (map (skolemize o make_nnf) hyps) 1
564 THEN REPEAT (etac exE 1))),
565 REPEAT_DETERM_N (length ts) o (etac thin_rl)] i st
567 handle Subscript => Seq.empty;
569 (*Top-level conversion to meta-level clauses. Each clause has
570 leading !!-bound universal variables, to express generality. To get
571 disjunctions instead of meta-clauses, remove "make_meta_clauses" below.*)
572 val make_clauses_tac =
575 let val ts = Logic.strip_assums_hyp prop
580 (map forall_intr_vars
581 (make_meta_clauses (make_clauses hyps))) 1)),
582 REPEAT_DETERM_N (length ts) o (etac thin_rl)]
586 (*** setup the special skoklemization methods ***)
588 (*No CHANGED_PROP here, since these always appear in the preamble*)
590 val skolemize_meth = Method.SIMPLE_METHOD' HEADGOAL skolemize_tac;
592 val make_clauses_meth = Method.SIMPLE_METHOD' HEADGOAL make_clauses_tac;
594 val skolemize_setup =
596 [("skolemize", Method.no_args skolemize_meth,
597 "Skolemization into existential quantifiers"),
598 ("make_clauses", Method.no_args make_clauses_meth,
599 "Conversion to !!-quantified meta-level clauses")];
603 structure BasicMeson: BASIC_MESON = Meson;