P. 14 of Theorem List - Intuitionistic Logic Explorer

Theorem List for Intuitionistic Logic Explorer - 1301-1400   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	3orbi123d 1301	Deduction joining 3 equivalences to form equivalence of disjunctions. (Contributed by NM, 20-Apr-1994.)
|- ( ph -> ( ps <-> ch ) )   &    |- ( ph -> ( th <-> ta ) )   &    |- ( ph -> ( et <-> ze ) )   =>    |- ( ph -> ( ( ps \/ th \/ et ) <-> ( ch \/ ta \/ ze ) ) )
Theorem	3anbi123d 1302	Deduction joining 3 equivalences to form equivalence of conjunctions. (Contributed by NM, 22-Apr-1994.)
|- ( ph -> ( ps <-> ch ) )   &    |- ( ph -> ( th <-> ta ) )   &    |- ( ph -> ( et <-> ze ) )   =>    |- ( ph -> ( ( ps /\ th /\ et ) <-> ( ch /\ ta /\ ze ) ) )
Theorem	3anbi12d 1303	Deduction conjoining and adding a conjunct to equivalences. (Contributed by NM, 8-Sep-2006.)
|- ( ph -> ( ps <-> ch ) )   &    |- ( ph -> ( th <-> ta ) )   =>    |- ( ph -> ( ( ps /\ th /\ et ) <-> ( ch /\ ta /\ et ) ) )
Theorem	3anbi13d 1304	Deduction conjoining and adding a conjunct to equivalences. (Contributed by NM, 8-Sep-2006.)
|- ( ph -> ( ps <-> ch ) )   &    |- ( ph -> ( th <-> ta ) )   =>    |- ( ph -> ( ( ps /\ et /\ th ) <-> ( ch /\ et /\ ta ) ) )
Theorem	3anbi23d 1305	Deduction conjoining and adding a conjunct to equivalences. (Contributed by NM, 8-Sep-2006.)
|- ( ph -> ( ps <-> ch ) )   &    |- ( ph -> ( th <-> ta ) )   =>    |- ( ph -> ( ( et /\ ps /\ th ) <-> ( et /\ ch /\ ta ) ) )
Theorem	3anbi1d 1306	Deduction adding conjuncts to an equivalence. (Contributed by NM, 8-Sep-2006.)
|- ( ph -> ( ps <-> ch ) )   =>    |- ( ph -> ( ( ps /\ th /\ ta ) <-> ( ch /\ th /\ ta ) ) )
Theorem	3anbi2d 1307	Deduction adding conjuncts to an equivalence. (Contributed by NM, 8-Sep-2006.)
|- ( ph -> ( ps <-> ch ) )   =>    |- ( ph -> ( ( th /\ ps /\ ta ) <-> ( th /\ ch /\ ta ) ) )
Theorem	3anbi3d 1308	Deduction adding conjuncts to an equivalence. (Contributed by NM, 8-Sep-2006.)
|- ( ph -> ( ps <-> ch ) )   =>    |- ( ph -> ( ( th /\ ta /\ ps ) <-> ( th /\ ta /\ ch ) ) )
Theorem	3anim123d 1309	Deduction joining 3 implications to form implication of conjunctions. (Contributed by NM, 24-Feb-2005.)
|- ( ph -> ( ps -> ch ) )   &    |- ( ph -> ( th -> ta ) )   &    |- ( ph -> ( et -> ze ) )   =>    |- ( ph -> ( ( ps /\ th /\ et ) -> ( ch /\ ta /\ ze ) ) )
Theorem	3orim123d 1310	Deduction joining 3 implications to form implication of disjunctions. (Contributed by NM, 4-Apr-1997.)
|- ( ph -> ( ps -> ch ) )   &    |- ( ph -> ( th -> ta ) )   &    |- ( ph -> ( et -> ze ) )   =>    |- ( ph -> ( ( ps \/ th \/ et ) -> ( ch \/ ta \/ ze ) ) )
Theorem	an6 1311	Rearrangement of 6 conjuncts. (Contributed by NM, 13-Mar-1995.)
|- ( ( ( ph /\ ps /\ ch ) /\ ( th /\ ta /\ et ) ) <-> ( ( ph /\ th ) /\ ( ps /\ ta ) /\ ( ch /\ et ) ) )
Theorem	3an6 1312	Analog of an4 576 for triple conjunction. (Contributed by Scott Fenton, 16-Mar-2011.) (Proof shortened by Andrew Salmon, 25-May-2011.)
|- ( ( ( ph /\ ps ) /\ ( ch /\ th ) /\ ( ta /\ et ) ) <-> ( ( ph /\ ch /\ ta ) /\ ( ps /\ th /\ et ) ) )
Theorem	3or6 1313	Analog of or4 761 for triple conjunction. (Contributed by Scott Fenton, 16-Mar-2011.)
|- ( ( ( ph \/ ps ) \/ ( ch \/ th ) \/ ( ta \/ et ) ) <-> ( ( ph \/ ch \/ ta ) \/ ( ps \/ th \/ et ) ) )
Theorem	mp3an1 1314	An inference based on modus ponens. (Contributed by NM, 21-Nov-1994.)
|- ph   &    |- ( ( ph /\ ps /\ ch ) -> th )   =>    |- ( ( ps /\ ch ) -> th )
Theorem	mp3an2 1315	An inference based on modus ponens. (Contributed by NM, 21-Nov-1994.)
|- ps   &    |- ( ( ph /\ ps /\ ch ) -> th )   =>    |- ( ( ph /\ ch ) -> th )
Theorem	mp3an3 1316	An inference based on modus ponens. (Contributed by NM, 21-Nov-1994.)
|- ch   &    |- ( ( ph /\ ps /\ ch ) -> th )   =>    |- ( ( ph /\ ps ) -> th )
Theorem	mp3an12 1317	An inference based on modus ponens. (Contributed by NM, 13-Jul-2005.)
|- ph   &    |- ps   &    |- ( ( ph /\ ps /\ ch ) -> th )   =>    |- ( ch -> th )
Theorem	mp3an13 1318	An inference based on modus ponens. (Contributed by NM, 14-Jul-2005.)
|- ph   &    |- ch   &    |- ( ( ph /\ ps /\ ch ) -> th )   =>    |- ( ps -> th )
Theorem	mp3an23 1319	An inference based on modus ponens. (Contributed by NM, 14-Jul-2005.)
|- ps   &    |- ch   &    |- ( ( ph /\ ps /\ ch ) -> th )   =>    |- ( ph -> th )
Theorem	mp3an1i 1320	An inference based on modus ponens. (Contributed by NM, 5-Jul-2005.)
|- ps   &    |- ( ph -> ( ( ps /\ ch /\ th ) -> ta ) )   =>    |- ( ph -> ( ( ch /\ th ) -> ta ) )
Theorem	mp3anl1 1321	An inference based on modus ponens. (Contributed by NM, 24-Feb-2005.)
|- ph   &    |- ( ( ( ph /\ ps /\ ch ) /\ th ) -> ta )   =>    |- ( ( ( ps /\ ch ) /\ th ) -> ta )
Theorem	mp3anl2 1322	An inference based on modus ponens. (Contributed by NM, 24-Feb-2005.)
|- ps   &    |- ( ( ( ph /\ ps /\ ch ) /\ th ) -> ta )   =>    |- ( ( ( ph /\ ch ) /\ th ) -> ta )
Theorem	mp3anl3 1323	An inference based on modus ponens. (Contributed by NM, 24-Feb-2005.)
|- ch   &    |- ( ( ( ph /\ ps /\ ch ) /\ th ) -> ta )   =>    |- ( ( ( ph /\ ps ) /\ th ) -> ta )
Theorem	mp3anr1 1324	An inference based on modus ponens. (Contributed by NM, 4-Nov-2006.)
|- ps   &    |- ( ( ph /\ ( ps /\ ch /\ th ) ) -> ta )   =>    |- ( ( ph /\ ( ch /\ th ) ) -> ta )
Theorem	mp3anr2 1325	An inference based on modus ponens. (Contributed by NM, 24-Nov-2006.)
|- ch   &    |- ( ( ph /\ ( ps /\ ch /\ th ) ) -> ta )   =>    |- ( ( ph /\ ( ps /\ th ) ) -> ta )
Theorem	mp3anr3 1326	An inference based on modus ponens. (Contributed by NM, 19-Oct-2007.)
|- th   &    |- ( ( ph /\ ( ps /\ ch /\ th ) ) -> ta )   =>    |- ( ( ph /\ ( ps /\ ch ) ) -> ta )
Theorem	mp3an 1327	An inference based on modus ponens. (Contributed by NM, 14-May-1999.)
|- ph   &    |- ps   &    |- ch   &    |- ( ( ph /\ ps /\ ch ) -> th )   =>    |- th
Theorem	mpd3an3 1328	An inference based on modus ponens. (Contributed by NM, 8-Nov-2007.)
|- ( ( ph /\ ps ) -> ch )   &    |- ( ( ph /\ ps /\ ch ) -> th )   =>    |- ( ( ph /\ ps ) -> th )
Theorem	mpd3an23 1329	An inference based on modus ponens. (Contributed by NM, 4-Dec-2006.)
|- ( ph -> ps )   &    |- ( ph -> ch )   &    |- ( ( ph /\ ps /\ ch ) -> th )   =>    |- ( ph -> th )
Theorem	mp3and 1330	A deduction based on modus ponens. (Contributed by Mario Carneiro, 24-Dec-2016.)
|- ( ph -> ps )   &    |- ( ph -> ch )   &    |- ( ph -> th )   &    |- ( ph -> ( ( ps /\ ch /\ th ) -> ta ) )   =>    |- ( ph -> ta )
Theorem	mp3an12i 1331	mp3an 1327 with antecedents in standard conjunction form and with one hypothesis an implication. (Contributed by Alan Sare, 28-Aug-2016.)
|- ph   &    |- ps   &    |- ( ch -> th )   &    |- ( ( ph /\ ps /\ th ) -> ta )   =>    |- ( ch -> ta )
Theorem	mp3an2i 1332	mp3an 1327 with antecedents in standard conjunction form and with two hypotheses which are implications. (Contributed by Alan Sare, 28-Aug-2016.)
|- ph   &    |- ( ps -> ch )   &    |- ( ps -> th )   &    |- ( ( ph /\ ch /\ th ) -> ta )   =>    |- ( ps -> ta )
Theorem	mp3an3an 1333	mp3an 1327 with antecedents in standard conjunction form and with two hypotheses which are implications. (Contributed by Alan Sare, 28-Aug-2016.)
|- ph   &    |- ( ps -> ch )   &    |- ( th -> ta )   &    |- ( ( ph /\ ch /\ ta ) -> et )   =>    |- ( ( ps /\ th ) -> et )
Theorem	mp3an2ani 1334	An elimination deduction. (Contributed by Alan Sare, 17-Oct-2017.)
|- ph   &    |- ( ps -> ch )   &    |- ( ( ps /\ th ) -> ta )   &    |- ( ( ph /\ ch /\ ta ) -> et )   =>    |- ( ( ps /\ th ) -> et )
Theorem	biimp3a 1335	Infer implication from a logical equivalence. Similar to biimpa 294. (Contributed by NM, 4-Sep-2005.)
|- ( ( ph /\ ps ) -> ( ch <-> th ) )   =>    |- ( ( ph /\ ps /\ ch ) -> th )
Theorem	biimp3ar 1336	Infer implication from a logical equivalence. Similar to biimpar 295. (Contributed by NM, 2-Jan-2009.)
|- ( ( ph /\ ps ) -> ( ch <-> th ) )   =>    |- ( ( ph /\ ps /\ th ) -> ch )
Theorem	3anandis 1337	Inference that undistributes a triple conjunction in the antecedent. (Contributed by NM, 18-Apr-2007.)
|- ( ( ( ph /\ ps ) /\ ( ph /\ ch ) /\ ( ph /\ th ) ) -> ta )   =>    |- ( ( ph /\ ( ps /\ ch /\ th ) ) -> ta )
Theorem	3anandirs 1338	Inference that undistributes a triple conjunction in the antecedent. (Contributed by NM, 25-Jul-2006.) (Revised by NM, 18-Apr-2007.)
|- ( ( ( ph /\ th ) /\ ( ps /\ th ) /\ ( ch /\ th ) ) -> ta )   =>    |- ( ( ( ph /\ ps /\ ch ) /\ th ) -> ta )
Theorem	ecased 1339	Deduction form of disjunctive syllogism. (Contributed by Jim Kingdon, 9-Dec-2017.)
|- ( ph -> -. ch )   &    |- ( ph -> ( ps \/ ch ) )   =>    |- ( ph -> ps )
Theorem	ecase23d 1340	Variation of ecased 1339 with three disjuncts instead of two. (Contributed by NM, 22-Apr-1994.) (Revised by Jim Kingdon, 9-Dec-2017.)
|- ( ph -> -. ch )   &    |- ( ph -> -. th )   &    |- ( ph -> ( ps \/ ch \/ th ) )   =>    |- ( ph -> ps )
1.2.12  True and false constants
1.2.12.1  Universal quantifier for use by df-tru Even though it is not ordinarily part of propositional calculus, the universal quantifier A. is introduced here so that the soundness of Definition df-tru 1346 can be checked by the same algorithm that is used for predicate calculus. Its first real use is in Axiom ax-5 1435 in the predicate calculus section below. For those who want propositional calculus to be self-contained, i.e., to use wff variables only, the alternate Definition dftru2 1351 may be adopted and this subsection moved down to the start of the subsection with wex 1480 below. However, the use of dftru2 1351 as a definition requires a more elaborate definition checking algorithm that we prefer to avoid.
Syntax	wal 1341	Extend wff definition to include the universal quantifier ("for all"). A. x ph is read " ph (phi) is true for all x". Typically, in its final application ph would be replaced with a wff containing a (free) occurrence of the variable x, for example x = y. In a universe with a finite number of objects, "for all" is equivalent to a big conjunction (AND) with one wff for each possible case of x. When the universe is infinite (as with set theory), such a propositional-calculus equivalent is not possible because an infinitely long formula has no meaning, but conceptually the idea is the same.
wff A. x ph
1.2.12.2  Equality predicate for use by df-tru Even though it is not ordinarily part of propositional calculus, the equality predicate = is introduced here so that the soundness of definition df-tru 1346 can be checked by the same algorithm as is used for predicate calculus. Its first real use is in Axiom ax-8 1492 in the predicate calculus section below. For those who want propositional calculus to be self-contained, i.e., to use wff variables only, the alternate definition dftru2 1351 may be adopted and this subsection moved down to just above weq 1491 below. However, the use of dftru2 1351 as a definition requires a more elaborate definition checking algorithm that we prefer to avoid.
Syntax	cv 1342	This syntax construction states that a variable x, which has been declared to be a setvar variable by $f statement vx, is also a class expression. This can be justified informally as follows. We know that the class builder { y | y e. x } is a class by cab 2151. Since (when y is distinct from x) we have x = { y | y e. x } by cvjust 2160, we can argue that the syntax " class x " can be viewed as an abbreviation for " class { y | y e. x }". See the discussion under the definition of class in [Jech] p. 4 showing that "Every set can be considered to be a class." While it is tempting and perhaps occasionally useful to view cv 1342 as a "type conversion" from a setvar variable to a class variable, keep in mind that cv 1342 is intrinsically no different from any other class-building syntax such as cab 2151, cun 3114, or c0 3409. For a general discussion of the theory of classes and the role of cv 1342, see https://us.metamath.org/mpeuni/mmset.html#class 1342. (The description above applies to set theory, not predicate calculus. The purpose of introducing class x here, and not in set theory where it belongs, is to allow us to express i.e. "prove" the weq 1491 of predicate calculus from the wceq 1343 of set theory, so that we don't overload the = connective with two syntax definitions. This is done to prevent ambiguity that would complicate some Metamath parsers.)
class x
Syntax	wceq 1343	Extend wff definition to include class equality. For a general discussion of the theory of classes, see https://us.metamath.org/mpeuni/mmset.html#class. (The purpose of introducing wff A = B here, and not in set theory where it belongs, is to allow us to express i.e. "prove" the weq 1491 of predicate calculus in terms of the wceq 1343 of set theory, so that we don't "overload" the = connective with two syntax definitions. This is done to prevent ambiguity that would complicate some Metamath parsers. For example, some parsers - although not the Metamath program - stumble on the fact that the = in x = y could be the = of either weq 1491 or wceq 1343, although mathematically it makes no difference. The class variables A and B are introduced temporarily for the purpose of this definition but otherwise not used in predicate calculus. See df-cleq 2158 for more information on the set theory usage of wceq 1343.)
wff A = B
1.2.12.3  Define the true and false constants
Syntax	wtru 1344	T. is a wff.
wff T.
Theorem	trujust 1345	Soundness justification theorem for df-tru 1346. (Contributed by Mario Carneiro, 17-Nov-2013.) (Revised by NM, 11-Jul-2019.)
|- ( ( A. x x = x -> A. x x = x ) <-> ( A. y y = y -> A. y y = y ) )
Definition	df-tru 1346	Definition of the truth value "true", or "verum", denoted by T.. This is a tautology, as proved by tru 1347. In this definition, an instance of id 19 is used as the definiens, although any tautology, such as an axiom, can be used in its place. This particular id 19 instance was chosen so this definition can be checked by the same algorithm that is used for predicate calculus. This definition should be referenced directly only by tru 1347, and other proofs should depend on tru 1347 (directly or indirectly) instead of this definition, since there are many alternate ways to define T.. (Contributed by Anthony Hart, 13-Oct-2010.) (Revised by NM, 11-Jul-2019.) (New usage is discouraged.)
|- ( T. <-> ( A. x x = x -> A. x x = x ) )
Theorem	tru 1347	The truth value T. is provable. (Contributed by Anthony Hart, 13-Oct-2010.)
|- T.
Syntax	wfal 1348	F. is a wff.
wff F.
Definition	df-fal 1349	Definition of the truth value "false", or "falsum", denoted by F.. See also df-tru 1346. (Contributed by Anthony Hart, 22-Oct-2010.)
|- ( F. <-> -. T. )
Theorem	fal 1350	The truth value F. is refutable. (Contributed by Anthony Hart, 22-Oct-2010.) (Proof shortened by Mel L. O'Cat, 11-Mar-2012.)
|- -. F.
Theorem	dftru2 1351	An alternate definition of "true". (Contributed by Anthony Hart, 13-Oct-2010.) (Revised by BJ, 12-Jul-2019.) (New usage is discouraged.)
|- ( T. <-> ( ph -> ph ) )
Theorem	mptru 1352	Eliminate T. as an antecedent. A proposition implied by T. is true. (Contributed by Mario Carneiro, 13-Mar-2014.)
|- ( T. -> ph )   =>    |- ph
Theorem	tbtru 1353	A proposition is equivalent to itself being equivalent to T.. (Contributed by Anthony Hart, 14-Aug-2011.)
|- ( ph <-> ( ph <-> T. ) )
Theorem	nbfal 1354	The negation of a proposition is equivalent to itself being equivalent to F.. (Contributed by Anthony Hart, 14-Aug-2011.)
|- ( -. ph <-> ( ph <-> F. ) )
Theorem	bitru 1355	A theorem is equivalent to truth. (Contributed by Mario Carneiro, 9-May-2015.)
|- ph   =>    |- ( ph <-> T. )
Theorem	bifal 1356	A contradiction is equivalent to falsehood. (Contributed by Mario Carneiro, 9-May-2015.)
|- -. ph   =>    |- ( ph <-> F. )
Theorem	falim 1357	The truth value F. implies anything. Also called the principle of explosion, or "ex falso quodlibet". (Contributed by FL, 20-Mar-2011.) (Proof shortened by Anthony Hart, 1-Aug-2011.)
|- ( F. -> ph )
Theorem	falimd 1358	The truth value F. implies anything. (Contributed by Mario Carneiro, 9-Feb-2017.)
|- ( ( ph /\ F. ) -> ps )
Theorem	a1tru 1359	Anything implies T.. (Contributed by FL, 20-Mar-2011.) (Proof shortened by Anthony Hart, 1-Aug-2011.)
|- ( ph -> T. )
Theorem	truan 1360	True can be removed from a conjunction. (Contributed by FL, 20-Mar-2011.) (Proof shortened by Wolf Lammen, 21-Jul-2019.)
|- ( ( T. /\ ph ) <-> ph )
Theorem	dfnot 1361	Given falsum, we can define the negation of a wff ph as the statement that a contradiction follows from assuming ph. (Contributed by Mario Carneiro, 9-Feb-2017.) (Proof shortened by Wolf Lammen, 21-Jul-2019.)
|- ( -. ph <-> ( ph -> F. ) )
Theorem	inegd 1362	Negation introduction rule from natural deduction. (Contributed by Mario Carneiro, 9-Feb-2017.)
|- ( ( ph /\ ps ) -> F. )   =>    |- ( ph -> -. ps )
Theorem	pm2.21fal 1363	If a wff and its negation are provable, then falsum is provable. (Contributed by Mario Carneiro, 9-Feb-2017.)
|- ( ph -> ps )   &    |- ( ph -> -. ps )   =>    |- ( ph -> F. )
Theorem	pclem6 1364	Negation inferred from embedded conjunct. (Contributed by NM, 20-Aug-1993.) (Proof rewritten by Jim Kingdon, 4-May-2018.)
|- ( ( ph <-> ( ps /\ -. ph ) ) -> -. ps )
1.2.13  Logical 'xor'
Syntax	wxo 1365	Extend wff definition to include exclusive disjunction ('xor').
wff ( ph \/_ ps )
Definition	df-xor 1366	Define exclusive disjunction (logical 'xor'). Return true if either the left or right, but not both, are true. Contrast with /\ (wa 103), \/ (wo 698), and -> (wi 4) . (Contributed by FL, 22-Nov-2010.) (Modified by Jim Kingdon, 1-Mar-2018.)
|- ( ( ph \/_ ps ) <-> ( ( ph \/ ps ) /\ -. ( ph /\ ps ) ) )
Theorem	xoranor 1367	One way of defining exclusive or. Equivalent to df-xor 1366. (Contributed by Jim Kingdon and Mario Carneiro, 1-Mar-2018.)
|- ( ( ph \/_ ps ) <-> ( ( ph \/ ps ) /\ ( -. ph \/ -. ps ) ) )
Theorem	excxor 1368	This tautology shows that xor is really exclusive. (Contributed by FL, 22-Nov-2010.) (Proof rewritten by Jim Kingdon, 5-May-2018.)
|- ( ( ph \/_ ps ) <-> ( ( ph /\ -. ps ) \/ ( -. ph /\ ps ) ) )
Theorem	xoror 1369	XOR implies OR. (Contributed by BJ, 19-Apr-2019.)
|- ( ( ph \/_ ps ) -> ( ph \/ ps ) )
Theorem	xorbi2d 1370	Deduction joining an equivalence and a left operand to form equivalence of exclusive-or. (Contributed by Jim Kingdon, 7-Oct-2018.)
|- ( ph -> ( ps <-> ch ) )   =>    |- ( ph -> ( ( th \/_ ps ) <-> ( th \/_ ch ) ) )
Theorem	xorbi1d 1371	Deduction joining an equivalence and a right operand to form equivalence of exclusive-or. (Contributed by Jim Kingdon, 7-Oct-2018.)
|- ( ph -> ( ps <-> ch ) )   =>    |- ( ph -> ( ( ps \/_ th ) <-> ( ch \/_ th ) ) )
Theorem	xorbi12d 1372	Deduction joining two equivalences to form equivalence of exclusive-or. (Contributed by Jim Kingdon, 7-Oct-2018.)
|- ( ph -> ( ps <-> ch ) )   &    |- ( ph -> ( th <-> ta ) )   =>    |- ( ph -> ( ( ps \/_ th ) <-> ( ch \/_ ta ) ) )
Theorem	xorbi12i 1373	Equality property for XOR. (Contributed by Mario Carneiro, 4-Sep-2016.)
|- ( ph <-> ps )   &    |- ( ch <-> th )   =>    |- ( ( ph \/_ ch ) <-> ( ps \/_ th ) )
Theorem	xorbin 1374	A consequence of exclusive or. In classical logic the converse also holds. (Contributed by Jim Kingdon, 8-Mar-2018.)
|- ( ( ph \/_ ps ) -> ( ph <-> -. ps ) )
Theorem	pm5.18im 1375	One direction of pm5.18dc 873, which holds for all propositions, not just decidable propositions. (Contributed by Jim Kingdon, 10-Mar-2018.)
|- ( ( ph <-> ps ) -> -. ( ph <-> -. ps ) )
Theorem	xornbi 1376	A consequence of exclusive or. For decidable propositions this is an equivalence, as seen at xornbidc 1381. (Contributed by Jim Kingdon, 10-Mar-2018.)
|- ( ( ph \/_ ps ) -> -. ( ph <-> ps ) )
Theorem	xor3dc 1377	Two ways to express "exclusive or" between decidable propositions. (Contributed by Jim Kingdon, 12-Apr-2018.)
|- (DECID ph -> (DECID ps -> ( -. ( ph <-> ps ) <-> ( ph <-> -. ps ) ) ) )
Theorem	xorcom 1378	\/_ is commutative. (Contributed by David A. Wheeler, 6-Oct-2018.)
|- ( ( ph \/_ ps ) <-> ( ps \/_ ph ) )
Theorem	pm5.15dc 1379	A decidable proposition is equivalent to a decidable proposition or its negation. Based on theorem *5.15 of [WhiteheadRussell] p. 124. (Contributed by Jim Kingdon, 18-Apr-2018.)
|- (DECID ph -> (DECID ps -> ( ( ph <-> ps ) \/ ( ph <-> -. ps ) ) ) )
Theorem	xor2dc 1380	Two ways to express "exclusive or" between decidable propositions. (Contributed by Jim Kingdon, 17-Apr-2018.)
|- (DECID ph -> (DECID ps -> ( -. ( ph <-> ps ) <-> ( ( ph \/ ps ) /\ -. ( ph /\ ps ) ) ) ) )
Theorem	xornbidc 1381	Exclusive or is equivalent to negated biconditional for decidable propositions. (Contributed by Jim Kingdon, 27-Apr-2018.)
|- (DECID ph -> (DECID ps -> ( ( ph \/_ ps ) <-> -. ( ph <-> ps ) ) ) )
Theorem	xordc 1382	Two ways to express "exclusive or" between decidable propositions. Theorem *5.22 of [WhiteheadRussell] p. 124, but for decidable propositions. (Contributed by Jim Kingdon, 5-May-2018.)
|- (DECID ph -> (DECID ps -> ( -. ( ph <-> ps ) <-> ( ( ph /\ -. ps ) \/ ( ps /\ -. ph ) ) ) ) )
Theorem	xordc1 1383	Exclusive or implies the left proposition is decidable. (Contributed by Jim Kingdon, 12-Mar-2018.)
|- ( ( ph \/_ ps ) -> DECID ph )
Theorem	nbbndc 1384	Move negation outside of biconditional, for decidable propositions. Compare Theorem *5.18 of [WhiteheadRussell] p. 124. (Contributed by Jim Kingdon, 18-Apr-2018.)
|- (DECID ph -> (DECID ps -> ( ( -. ph <-> ps ) <-> -. ( ph <-> ps ) ) ) )
Theorem	biassdc 1385	Associative law for the biconditional, for decidable propositions. The classical version (without the decidability conditions) is an axiom of system DS in Vladimir Lifschitz, "On calculational proofs", Annals of Pure and Applied Logic, 113:207-224, 2002, http://www.cs.utexas.edu/users/ai-lab/pub-view.php?PubID=26805, and, interestingly, was not included in Principia Mathematica but was apparently first noted by Jan Lukasiewicz circa 1923. (Contributed by Jim Kingdon, 4-May-2018.)
|- (DECID ph -> (DECID ps -> (DECID ch -> ( ( ( ph <-> ps ) <-> ch ) <-> ( ph <-> ( ps <-> ch ) ) ) ) ) )
Theorem	bilukdc 1386	Lukasiewicz's shortest axiom for equivalential calculus (but modified to require decidable propositions). Storrs McCall, ed., Polish Logic 1920-1939 (Oxford, 1967), p. 96. (Contributed by Jim Kingdon, 5-May-2018.)
|- ( ( (DECID ph /\ DECID ps ) /\ DECID ch ) -> ( ( ph <-> ps ) <-> ( ( ch <-> ps ) <-> ( ph <-> ch ) ) ) )
Theorem	dfbi3dc 1387	An alternate definition of the biconditional for decidable propositions. Theorem *5.23 of [WhiteheadRussell] p. 124, but with decidability conditions. (Contributed by Jim Kingdon, 5-May-2018.)
|- (DECID ph -> (DECID ps -> ( ( ph <-> ps ) <-> ( ( ph /\ ps ) \/ ( -. ph /\ -. ps ) ) ) ) )
Theorem	pm5.24dc 1388	Theorem *5.24 of [WhiteheadRussell] p. 124, but for decidable propositions. (Contributed by Jim Kingdon, 5-May-2018.)
|- (DECID ph -> (DECID ps -> ( -. ( ( ph /\ ps ) \/ ( -. ph /\ -. ps ) ) <-> ( ( ph /\ -. ps ) \/ ( ps /\ -. ph ) ) ) ) )
Theorem	xordidc 1389	Conjunction distributes over exclusive-or, for decidable propositions. This is one way to interpret the distributive law of multiplication over addition in modulo 2 arithmetic. (Contributed by Jim Kingdon, 14-Jul-2018.)
|- (DECID ph -> (DECID ps -> (DECID ch -> ( ( ph /\ ( ps \/_ ch ) ) <-> ( ( ph /\ ps ) \/_ ( ph /\ ch ) ) ) ) ) )
Theorem	anxordi 1390	Conjunction distributes over exclusive-or. (Contributed by Mario Carneiro and Jim Kingdon, 7-Oct-2018.)
|- ( ( ph /\ ( ps \/_ ch ) ) <-> ( ( ph /\ ps ) \/_ ( ph /\ ch ) ) )
1.2.14  Truth tables: Operations on true and false constants For classical logic, truth tables can be used to define propositional logic operations, by showing the results of those operations for all possible combinations of true (T.) and false (F.). Although the intuitionistic logic connectives are not as simply defined, T. and F. do play similar roles as in classical logic and most theorems from classical logic continue to hold. Here we show that our definitions and axioms produce equivalent results for T. and F. as we would get from truth tables for /\ (conjunction aka logical 'and') wa 103, \/ (disjunction aka logical inclusive 'or') wo 698, -> (implies) wi 4, -. (not) wn 3, <-> (logical equivalence) df-bi 116, and \/_ (exclusive or) df-xor 1366.
Theorem	truantru 1391	A /\ identity. (Contributed by Anthony Hart, 22-Oct-2010.)
|- ( ( T. /\ T. ) <-> T. )
Theorem	truanfal 1392	A /\ identity. (Contributed by Anthony Hart, 22-Oct-2010.)
|- ( ( T. /\ F. ) <-> F. )
Theorem	falantru 1393	A /\ identity. (Contributed by David A. Wheeler, 23-Feb-2018.)
|- ( ( F. /\ T. ) <-> F. )
Theorem	falanfal 1394	A /\ identity. (Contributed by Anthony Hart, 22-Oct-2010.)
|- ( ( F. /\ F. ) <-> F. )
Theorem	truortru 1395	A \/ identity. (Contributed by Anthony Hart, 22-Oct-2010.) (Proof shortened by Andrew Salmon, 13-May-2011.)
|- ( ( T. \/ T. ) <-> T. )
Theorem	truorfal 1396	A \/ identity. (Contributed by Anthony Hart, 22-Oct-2010.)
|- ( ( T. \/ F. ) <-> T. )
Theorem	falortru 1397	A \/ identity. (Contributed by Anthony Hart, 22-Oct-2010.)
|- ( ( F. \/ T. ) <-> T. )
Theorem	falorfal 1398	A \/ identity. (Contributed by Anthony Hart, 22-Oct-2010.) (Proof shortened by Andrew Salmon, 13-May-2011.)
|- ( ( F. \/ F. ) <-> F. )
Theorem	truimtru 1399	A -> identity. (Contributed by Anthony Hart, 22-Oct-2010.)
|- ( ( T. -> T. ) <-> T. )
Theorem	truimfal 1400	A -> identity. (Contributed by Anthony Hart, 22-Oct-2010.) (Proof shortened by Andrew Salmon, 13-May-2011.)
|- ( ( T. -> F. ) <-> F. )