P. 43 of Theorem List - Intuitionistic Logic Explorer

Theorem List for Intuitionistic Logic Explorer - 4201-4300   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	pwntru 4201	A slight strengthening of pwtrufal 14832. (Contributed by Mario Carneiro and Jim Kingdon, 12-Sep-2023.)
|- ( ( A C_ { (/) } /\ A =/= { (/) } ) -> A = (/) )
Theorem	exmid1dc 4202*	A convenience theorem for proving that something implies EXMID. Think of this as an alternative to using a proposition, as in proofs like undifexmid 4195 or ordtriexmid 4522. In this context x = { (/) } can be thought of as "x is true". (Contributed by Jim Kingdon, 21-Nov-2023.)
|- ( ( ph /\ x C_ { (/) } ) -> DECID x = { (/) } )   =>    |- ( ph -> EXMID )
Theorem	exmidn0m 4203*	Excluded middle is equivalent to any set being empty or inhabited. (Contributed by Jim Kingdon, 5-Mar-2023.)
|- (EXMID <-> A. x ( x = (/) \/ E. y y e. x ) )
Theorem	exmidsssn 4204*	Excluded middle is equivalent to the biconditionalized version of sssnr 3755 for sets. (Contributed by Jim Kingdon, 5-Mar-2023.)
|- (EXMID <-> A. x A. y ( x C_ { y } <-> ( x = (/) \/ x = { y } ) ) )
Theorem	exmidsssnc 4205*	Excluded middle in terms of subsets of a singleton. This is similar to exmid01 4200 but lets you choose any set as the element of the singleton rather than just (/). It is similar to exmidsssn 4204 but for a particular set B rather than all sets. (Contributed by Jim Kingdon, 29-Jul-2023.)
|- ( B e. V -> (EXMID <-> A. x ( x C_ { B } -> ( x = (/) \/ x = { B } ) ) ) )
Theorem	exmid0el 4206	Excluded middle is equivalent to decidability of (/) being an element of an arbitrary set. (Contributed by Jim Kingdon, 18-Jun-2022.)
|- (EXMID <-> A. xDECID (/) e. x )
Theorem	exmidel 4207*	Excluded middle is equivalent to decidability of membership for two arbitrary sets. (Contributed by Jim Kingdon, 18-Jun-2022.)
|- (EXMID <-> A. x A. yDECID x e. y )
Theorem	exmidundif 4208*	Excluded middle is equivalent to every subset having a complement. That is, the union of a subset and its relative complement being the whole set. Although special cases such as undifss 3505 and undifdcss 6924 are provable, the full statement is equivalent to excluded middle as shown here. (Contributed by Jim Kingdon, 18-Jun-2022.)
|- (EXMID <-> A. x A. y ( x C_ y <-> ( x u. ( y \ x ) ) = y ) )
Theorem	exmidundifim 4209*	Excluded middle is equivalent to every subset having a complement. Variation of exmidundif 4208 with an implication rather than a biconditional. (Contributed by Jim Kingdon, 16-Feb-2023.)
|- (EXMID <-> A. x A. y ( x C_ y -> ( x u. ( y \ x ) ) = y ) )
Theorem	exmid1stab 4210*	If every proposition is stable, excluded middle follows. We are thinking of x as a proposition and x = { (/) } as " x is true". (Contributed by Jim Kingdon, 28-Nov-2023.)
|- ( ( ph /\ x C_ { (/) } ) -> STAB x = { (/) } )   =>    |- ( ph -> EXMID )
2.3.3  Axiom of Pairing
Axiom	ax-pr 4211*	The Axiom of Pairing of IZF set theory. Axiom 2 of [Crosilla] p. "Axioms of CZF and IZF", except (a) unnecessary quantifiers are removed, and (b) Crosilla has a biconditional rather than an implication (but the two are equivalent by bm1.3ii 4126). (Contributed by NM, 14-Nov-2006.)
|- E. z A. w ( ( w = x \/ w = y ) -> w e. z )
Theorem	zfpair2 4212	Derive the abbreviated version of the Axiom of Pairing from ax-pr 4211. (Contributed by NM, 14-Nov-2006.)
|- { x , y } e. _V
Theorem	prexg 4213	The Axiom of Pairing using class variables. Theorem 7.13 of [Quine] p. 51, but restricted to classes which exist. For proper classes, see prprc 3704, prprc1 3702, and prprc2 3703. (Contributed by Jim Kingdon, 16-Sep-2018.)
|- ( ( A e. V /\ B e. W ) -> { A , B } e. _V )
Theorem	snelpwi 4214	A singleton of a set belongs to the power class of a class containing the set. (Contributed by Alan Sare, 25-Aug-2011.)
|- ( A e. B -> { A } e. ~P B )
Theorem	snelpw 4215	A singleton of a set belongs to the power class of a class containing the set. (Contributed by NM, 1-Apr-1998.)
|- A e. _V   =>    |- ( A e. B <-> { A } e. ~P B )
Theorem	prelpwi 4216	A pair of two sets belongs to the power class of a class containing those two sets. (Contributed by Thierry Arnoux, 10-Mar-2017.)
|- ( ( A e. C /\ B e. C ) -> { A , B } e. ~P C )
Theorem	rext 4217*	A theorem similar to extensionality, requiring the existence of a singleton. Exercise 8 of [TakeutiZaring] p. 16. (Contributed by NM, 10-Aug-1993.)
|- ( A. z ( x e. z -> y e. z ) -> x = y )
Theorem	sspwb 4218	Classes are subclasses if and only if their power classes are subclasses. Exercise 18 of [TakeutiZaring] p. 18. (Contributed by NM, 13-Oct-1996.)
|- ( A C_ B <-> ~P A C_ ~P B )
Theorem	unipw 4219	A class equals the union of its power class. Exercise 6(a) of [Enderton] p. 38. (Contributed by NM, 14-Oct-1996.) (Proof shortened by Alan Sare, 28-Dec-2008.)
|- U. ~P A = A
Theorem	pwel 4220	Membership of a power class. Exercise 10 of [Enderton] p. 26. (Contributed by NM, 13-Jan-2007.)
|- ( A e. B -> ~P A e. ~P ~P U. B )
Theorem	pwtr 4221	A class is transitive iff its power class is transitive. (Contributed by Alan Sare, 25-Aug-2011.) (Revised by Mario Carneiro, 15-Jun-2014.)
|- ( Tr A <-> Tr ~P A )
Theorem	ssextss 4222*	An extensionality-like principle defining subclass in terms of subsets. (Contributed by NM, 30-Jun-2004.)
|- ( A C_ B <-> A. x ( x C_ A -> x C_ B ) )
Theorem	ssext 4223*	An extensionality-like principle that uses the subset instead of the membership relation: two classes are equal iff they have the same subsets. (Contributed by NM, 30-Jun-2004.)
|- ( A = B <-> A. x ( x C_ A <-> x C_ B ) )
Theorem	nssssr 4224*	Negation of subclass relationship. Compare nssr 3217. (Contributed by Jim Kingdon, 17-Sep-2018.)
|- ( E. x ( x C_ A /\ -. x C_ B ) -> -. A C_ B )
Theorem	pweqb 4225	Classes are equal if and only if their power classes are equal. Exercise 19 of [TakeutiZaring] p. 18. (Contributed by NM, 13-Oct-1996.)
|- ( A = B <-> ~P A = ~P B )
Theorem	intid 4226*	The intersection of all sets to which a set belongs is the singleton of that set. (Contributed by NM, 5-Jun-2009.)
|- A e. _V   =>    |- |^| { x | A e. x } = { A }
Theorem	euabex 4227	The abstraction of a wff with existential uniqueness exists. (Contributed by NM, 25-Nov-1994.)
|- ( E! x ph -> { x | ph } e. _V )
Theorem	mss 4228*	An inhabited class (even if proper) has an inhabited subset. (Contributed by Jim Kingdon, 17-Sep-2018.)
|- ( E. y y e. A -> E. x ( x C_ A /\ E. z z e. x ) )
Theorem	exss 4229*	Restricted existence in a class (even if proper) implies restricted existence in a subset. (Contributed by NM, 23-Aug-2003.)
|- ( E. x e. A ph -> E. y ( y C_ A /\ E. x e. y ph ) )
Theorem	opexg 4230	An ordered pair of sets is a set. (Contributed by Jim Kingdon, 11-Jan-2019.)
|- ( ( A e. V /\ B e. W ) -> <. A , B >. e. _V )
Theorem	opex 4231	An ordered pair of sets is a set. (Contributed by Jim Kingdon, 24-Sep-2018.) (Revised by Mario Carneiro, 24-May-2019.)
|- A e. _V   &    |- B e. _V   =>    |- <. A , B >. e. _V
Theorem	otexg 4232	An ordered triple of sets is a set. (Contributed by Jim Kingdon, 19-Sep-2018.)
|- ( ( A e. U /\ B e. V /\ C e. W ) -> <. A , B , C >. e. _V )
Theorem	elop 4233	An ordered pair has two elements. Exercise 3 of [TakeutiZaring] p. 15. (Contributed by NM, 5-Aug-1993.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- A e. _V   &    |- B e. _V   &    |- C e. _V   =>    |- ( A e. <. B , C >. <-> ( A = { B } \/ A = { B , C } ) )
Theorem	opi1 4234	One of the two elements in an ordered pair. (Contributed by NM, 5-Aug-1993.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- A e. _V   &    |- B e. _V   =>    |- { A } e. <. A , B >.
Theorem	opi2 4235	One of the two elements of an ordered pair. (Contributed by NM, 5-Aug-1993.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- A e. _V   &    |- B e. _V   =>    |- { A , B } e. <. A , B >.
2.3.4  Ordered pair theorem
Theorem	opm 4236*	An ordered pair is inhabited iff the arguments are sets. (Contributed by Jim Kingdon, 21-Sep-2018.)
|- ( E. x x e. <. A , B >. <-> ( A e. _V /\ B e. _V ) )
Theorem	opnzi 4237	An ordered pair is nonempty if the arguments are sets (it is also inhabited; see opm 4236). (Contributed by Mario Carneiro, 26-Apr-2015.)
|- A e. _V   &    |- B e. _V   =>    |- <. A , B >. =/= (/)
Theorem	opth1 4238	Equality of the first members of equal ordered pairs. (Contributed by NM, 28-May-2008.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- A e. _V   &    |- B e. _V   =>    |- ( <. A , B >. = <. C , D >. -> A = C )
Theorem	opth 4239	The ordered pair theorem. If two ordered pairs are equal, their first elements are equal and their second elements are equal. Exercise 6 of [TakeutiZaring] p. 16. Note that C and D are not required to be sets due our specific ordered pair definition. (Contributed by NM, 28-May-1995.)
|- A e. _V   &    |- B e. _V   =>    |- ( <. A , B >. = <. C , D >. <-> ( A = C /\ B = D ) )
Theorem	opthg 4240	Ordered pair theorem. C and D are not required to be sets under our specific ordered pair definition. (Contributed by NM, 14-Oct-2005.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- ( ( A e. V /\ B e. W ) -> ( <. A , B >. = <. C , D >. <-> ( A = C /\ B = D ) ) )
Theorem	opthg2 4241	Ordered pair theorem. (Contributed by NM, 14-Oct-2005.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- ( ( C e. V /\ D e. W ) -> ( <. A , B >. = <. C , D >. <-> ( A = C /\ B = D ) ) )
Theorem	opth2 4242	Ordered pair theorem. (Contributed by NM, 21-Sep-2014.)
|- C e. _V   &    |- D e. _V   =>    |- ( <. A , B >. = <. C , D >. <-> ( A = C /\ B = D ) )
Theorem	otth2 4243	Ordered triple theorem, with triple express with ordered pairs. (Contributed by NM, 1-May-1995.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- A e. _V   &    |- B e. _V   &    |- R e. _V   =>    |- ( <. <. A , B >. , R >. = <. <. C , D >. , S >. <-> ( A = C /\ B = D /\ R = S ) )
Theorem	otth 4244	Ordered triple theorem. (Contributed by NM, 25-Sep-2014.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- A e. _V   &    |- B e. _V   &    |- R e. _V   =>    |- ( <. A , B , R >. = <. C , D , S >. <-> ( A = C /\ B = D /\ R = S ) )
Theorem	eqvinop 4245*	A variable introduction law for ordered pairs. Analog of Lemma 15 of [Monk2] p. 109. (Contributed by NM, 28-May-1995.)
|- B e. _V   &    |- C e. _V   =>    |- ( A = <. B , C >. <-> E. x E. y ( A = <. x , y >. /\ <. x , y >. = <. B , C >. ) )
Theorem	copsexg 4246*	Substitution of class A for ordered pair <. x , y >.. (Contributed by NM, 27-Dec-1996.) (Revised by Andrew Salmon, 11-Jul-2011.)
|- ( A = <. x , y >. -> ( ph <-> E. x E. y ( A = <. x , y >. /\ ph ) ) )
Theorem	copsex2t 4247*	Closed theorem form of copsex2g 4248. (Contributed by NM, 17-Feb-2013.)
|- ( ( A. x A. y ( ( x = A /\ y = B ) -> ( ph <-> ps ) ) /\ ( A e. V /\ B e. W ) ) -> ( E. x E. y ( <. A , B >. = <. x , y >. /\ ph ) <-> ps ) )
Theorem	copsex2g 4248*	Implicit substitution inference for ordered pairs. (Contributed by NM, 28-May-1995.)
|- ( ( x = A /\ y = B ) -> ( ph <-> ps ) )   =>    |- ( ( A e. V /\ B e. W ) -> ( E. x E. y ( <. A , B >. = <. x , y >. /\ ph ) <-> ps ) )
Theorem	copsex4g 4249*	An implicit substitution inference for 2 ordered pairs. (Contributed by NM, 5-Aug-1995.)
|- ( ( ( x = A /\ y = B ) /\ ( z = C /\ w = D ) ) -> ( ph <-> ps ) )   =>    |- ( ( ( A e. R /\ B e. S ) /\ ( C e. R /\ D e. S ) ) -> ( E. x E. y E. z E. w ( ( <. A , B >. = <. x , y >. /\ <. C , D >. = <. z , w >. ) /\ ph ) <-> ps ) )
Theorem	0nelop 4250	A property of ordered pairs. (Contributed by Mario Carneiro, 26-Apr-2015.)
|- -. (/) e. <. A , B >.
Theorem	opeqex 4251	Equivalence of existence implied by equality of ordered pairs. (Contributed by NM, 28-May-2008.)
|- ( <. A , B >. = <. C , D >. -> ( ( A e. _V /\ B e. _V ) <-> ( C e. _V /\ D e. _V ) ) )
Theorem	opcom 4252	An ordered pair commutes iff its members are equal. (Contributed by NM, 28-May-2009.)
|- A e. _V   &    |- B e. _V   =>    |- ( <. A , B >. = <. B , A >. <-> A = B )
Theorem	moop2 4253*	"At most one" property of an ordered pair. (Contributed by NM, 11-Apr-2004.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- B e. _V   =>    |- E* x A = <. B , x >.
Theorem	opeqsn 4254	Equivalence for an ordered pair equal to a singleton. (Contributed by NM, 3-Jun-2008.)
|- A e. _V   &    |- B e. _V   &    |- C e. _V   =>    |- ( <. A , B >. = { C } <-> ( A = B /\ C = { A } ) )
Theorem	opeqpr 4255	Equivalence for an ordered pair equal to an unordered pair. (Contributed by NM, 3-Jun-2008.)
|- A e. _V   &    |- B e. _V   &    |- C e. _V   &    |- D e. _V   =>    |- ( <. A , B >. = { C , D } <-> ( ( C = { A } /\ D = { A , B } ) \/ ( C = { A , B } /\ D = { A } ) ) )
Theorem	euotd 4256*	Prove existential uniqueness for an ordered triple. (Contributed by Mario Carneiro, 20-May-2015.)
|- ( ph -> A e. _V )   &    |- ( ph -> B e. _V )   &    |- ( ph -> C e. _V )   &    |- ( ph -> ( ps <-> ( a = A /\ b = B /\ c = C ) ) )   =>    |- ( ph -> E! x E. a E. b E. c ( x = <. a , b , c >. /\ ps ) )
Theorem	uniop 4257	The union of an ordered pair. Theorem 65 of [Suppes] p. 39. (Contributed by NM, 17-Aug-2004.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- A e. _V   &    |- B e. _V   =>    |- U. <. A , B >. = { A , B }
Theorem	uniopel 4258	Ordered pair membership is inherited by class union. (Contributed by NM, 13-May-2008.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- A e. _V   &    |- B e. _V   =>    |- ( <. A , B >. e. C -> U. <. A , B >. e. U. C )
2.3.5  Ordered-pair class abstractions (cont.)
Theorem	opabid 4259	The law of concretion. Special case of Theorem 9.5 of [Quine] p. 61. (Contributed by NM, 14-Apr-1995.) (Proof shortened by Andrew Salmon, 25-Jul-2011.)
|- ( <. x , y >. e. { <. x , y >. | ph } <-> ph )
Theorem	elopab 4260*	Membership in a class abstraction of ordered pairs. (Contributed by NM, 24-Mar-1998.)
|- ( A e. { <. x , y >. | ph } <-> E. x E. y ( A = <. x , y >. /\ ph ) )
Theorem	opelopabsbALT 4261*	The law of concretion in terms of substitutions. Less general than opelopabsb 4262, but having a much shorter proof. (Contributed by NM, 30-Sep-2002.) (Proof shortened by Andrew Salmon, 25-Jul-2011.) (New usage is discouraged.) (Proof modification is discouraged.)
|- ( <. z , w >. e. { <. x , y >. | ph } <-> [ w / y ] [ z / x ] ph )
Theorem	opelopabsb 4262*	The law of concretion in terms of substitutions. (Contributed by NM, 30-Sep-2002.) (Revised by Mario Carneiro, 18-Nov-2016.)
|- ( <. A , B >. e. { <. x , y >. | ph } <-> [. A / x ]. [. B / y ]. ph )
Theorem	brabsb 4263*	The law of concretion in terms of substitutions. (Contributed by NM, 17-Mar-2008.)
|- R = { <. x , y >. | ph }   =>    |- ( A R B <-> [. A / x ]. [. B / y ]. ph )
Theorem	opelopabt 4264*	Closed theorem form of opelopab 4273. (Contributed by NM, 19-Feb-2013.)
|- ( ( A. x A. y ( x = A -> ( ph <-> ps ) ) /\ A. x A. y ( y = B -> ( ps <-> ch ) ) /\ ( A e. V /\ B e. W ) ) -> ( <. A , B >. e. { <. x , y >. | ph } <-> ch ) )
Theorem	opelopabga 4265*	The law of concretion. Theorem 9.5 of [Quine] p. 61. (Contributed by Mario Carneiro, 19-Dec-2013.)
|- ( ( x = A /\ y = B ) -> ( ph <-> ps ) )   =>    |- ( ( A e. V /\ B e. W ) -> ( <. A , B >. e. { <. x , y >. | ph } <-> ps ) )
Theorem	brabga 4266*	The law of concretion for a binary relation. (Contributed by Mario Carneiro, 19-Dec-2013.)
|- ( ( x = A /\ y = B ) -> ( ph <-> ps ) )   &    |- R = { <. x , y >. | ph }   =>    |- ( ( A e. V /\ B e. W ) -> ( A R B <-> ps ) )
Theorem	opelopab2a 4267*	Ordered pair membership in an ordered pair class abstraction. (Contributed by Mario Carneiro, 19-Dec-2013.)
|- ( ( x = A /\ y = B ) -> ( ph <-> ps ) )   =>    |- ( ( A e. C /\ B e. D ) -> ( <. A , B >. e. { <. x , y >. | ( ( x e. C /\ y e. D ) /\ ph ) } <-> ps ) )
Theorem	opelopaba 4268*	The law of concretion. Theorem 9.5 of [Quine] p. 61. (Contributed by Mario Carneiro, 19-Dec-2013.)
|- A e. _V   &    |- B e. _V   &    |- ( ( x = A /\ y = B ) -> ( ph <-> ps ) )   =>    |- ( <. A , B >. e. { <. x , y >. | ph } <-> ps )
Theorem	braba 4269*	The law of concretion for a binary relation. (Contributed by NM, 19-Dec-2013.)
|- A e. _V   &    |- B e. _V   &    |- ( ( x = A /\ y = B ) -> ( ph <-> ps ) )   &    |- R = { <. x , y >. | ph }   =>    |- ( A R B <-> ps )
Theorem	opelopabg 4270*	The law of concretion. Theorem 9.5 of [Quine] p. 61. (Contributed by NM, 28-May-1995.) (Revised by Mario Carneiro, 19-Dec-2013.)
|- ( x = A -> ( ph <-> ps ) )   &    |- ( y = B -> ( ps <-> ch ) )   =>    |- ( ( A e. V /\ B e. W ) -> ( <. A , B >. e. { <. x , y >. | ph } <-> ch ) )
Theorem	brabg 4271*	The law of concretion for a binary relation. (Contributed by NM, 16-Aug-1999.) (Revised by Mario Carneiro, 19-Dec-2013.)
|- ( x = A -> ( ph <-> ps ) )   &    |- ( y = B -> ( ps <-> ch ) )   &    |- R = { <. x , y >. | ph }   =>    |- ( ( A e. C /\ B e. D ) -> ( A R B <-> ch ) )
Theorem	opelopab2 4272*	Ordered pair membership in an ordered pair class abstraction. (Contributed by NM, 14-Oct-2007.) (Revised by Mario Carneiro, 19-Dec-2013.)
|- ( x = A -> ( ph <-> ps ) )   &    |- ( y = B -> ( ps <-> ch ) )   =>    |- ( ( A e. C /\ B e. D ) -> ( <. A , B >. e. { <. x , y >. | ( ( x e. C /\ y e. D ) /\ ph ) } <-> ch ) )
Theorem	opelopab 4273*	The law of concretion. Theorem 9.5 of [Quine] p. 61. (Contributed by NM, 16-May-1995.)
|- A e. _V   &    |- B e. _V   &    |- ( x = A -> ( ph <-> ps ) )   &    |- ( y = B -> ( ps <-> ch ) )   =>    |- ( <. A , B >. e. { <. x , y >. | ph } <-> ch )
Theorem	brab 4274*	The law of concretion for a binary relation. (Contributed by NM, 16-Aug-1999.)
|- A e. _V   &    |- B e. _V   &    |- ( x = A -> ( ph <-> ps ) )   &    |- ( y = B -> ( ps <-> ch ) )   &    |- R = { <. x , y >. | ph }   =>    |- ( A R B <-> ch )
Theorem	opelopabaf 4275*	The law of concretion. Theorem 9.5 of [Quine] p. 61. This version of opelopab 4273 uses bound-variable hypotheses in place of distinct variable conditions. (Contributed by Mario Carneiro, 19-Dec-2013.) (Proof shortened by Mario Carneiro, 18-Nov-2016.)
|- F/ x ps   &    |- F/ y ps   &    |- A e. _V   &    |- B e. _V   &    |- ( ( x = A /\ y = B ) -> ( ph <-> ps ) )   =>    |- ( <. A , B >. e. { <. x , y >. | ph } <-> ps )
Theorem	opelopabf 4276*	The law of concretion. Theorem 9.5 of [Quine] p. 61. This version of opelopab 4273 uses bound-variable hypotheses in place of distinct variable conditions. (Contributed by NM, 19-Dec-2008.)
|- F/ x ps   &    |- F/ y ch   &    |- A e. _V   &    |- B e. _V   &    |- ( x = A -> ( ph <-> ps ) )   &    |- ( y = B -> ( ps <-> ch ) )   =>    |- ( <. A , B >. e. { <. x , y >. | ph } <-> ch )
Theorem	ssopab2 4277	Equivalence of ordered pair abstraction subclass and implication. (Contributed by NM, 27-Dec-1996.) (Revised by Mario Carneiro, 19-May-2013.)
|- ( A. x A. y ( ph -> ps ) -> { <. x , y >. | ph } C_ { <. x , y >. | ps } )
Theorem	ssopab2b 4278	Equivalence of ordered pair abstraction subclass and implication. (Contributed by NM, 27-Dec-1996.) (Proof shortened by Mario Carneiro, 18-Nov-2016.)
|- ( { <. x , y >. | ph } C_ { <. x , y >. | ps } <-> A. x A. y ( ph -> ps ) )
Theorem	ssopab2i 4279	Inference of ordered pair abstraction subclass from implication. (Contributed by NM, 5-Apr-1995.)
|- ( ph -> ps )   =>    |- { <. x , y >. | ph } C_ { <. x , y >. | ps }
Theorem	ssopab2dv 4280*	Inference of ordered pair abstraction subclass from implication. (Contributed by NM, 19-Jan-2014.) (Revised by Mario Carneiro, 24-Jun-2014.)
|- ( ph -> ( ps -> ch ) )   =>    |- ( ph -> { <. x , y >. | ps } C_ { <. x , y >. | ch } )
Theorem	eqopab2b 4281	Equivalence of ordered pair abstraction equality and biconditional. (Contributed by Mario Carneiro, 4-Jan-2017.)
|- ( { <. x , y >. | ph } = { <. x , y >. | ps } <-> A. x A. y ( ph <-> ps ) )
Theorem	opabm 4282*	Inhabited ordered pair class abstraction. (Contributed by Jim Kingdon, 29-Sep-2018.)
|- ( E. z z e. { <. x , y >. | ph } <-> E. x E. y ph )
Theorem	iunopab 4283*	Move indexed union inside an ordered-pair abstraction. (Contributed by Stefan O'Rear, 20-Feb-2015.)
|- U_ z e. A { <. x , y >. | ph } = { <. x , y >. | E. z e. A ph }
2.3.6  Power class of union and intersection
Theorem	pwin 4284	The power class of the intersection of two classes is the intersection of their power classes. Exercise 4.12(j) of [Mendelson] p. 235. (Contributed by NM, 23-Nov-2003.)
|- ~P ( A i^i B ) = ( ~P A i^i ~P B )
Theorem	pwunss 4285	The power class of the union of two classes includes the union of their power classes. Exercise 4.12(k) of [Mendelson] p. 235. (Contributed by NM, 23-Nov-2003.)
|- ( ~P A u. ~P B ) C_ ~P ( A u. B )
Theorem	pwssunim 4286	The power class of the union of two classes is a subset of the union of their power classes, if one class is a subclass of the other. One direction of Exercise 4.12(l) of [Mendelson] p. 235. (Contributed by Jim Kingdon, 30-Sep-2018.)
|- ( ( A C_ B \/ B C_ A ) -> ~P ( A u. B ) C_ ( ~P A u. ~P B ) )
Theorem	pwundifss 4287	Break up the power class of a union into a union of smaller classes. (Contributed by Jim Kingdon, 30-Sep-2018.)
|- ( ( ~P ( A u. B ) \ ~P A ) u. ~P A ) C_ ~P ( A u. B )
Theorem	pwunim 4288	The power class of the union of two classes equals the union of their power classes, iff one class is a subclass of the other. Part of Exercise 7(b) of [Enderton] p. 28. (Contributed by Jim Kingdon, 30-Sep-2018.)
|- ( ( A C_ B \/ B C_ A ) -> ~P ( A u. B ) = ( ~P A u. ~P B ) )
2.3.7  Epsilon and identity relations
Syntax	cep 4289	Extend class notation to include the epsilon relation.
class _E
Syntax	cid 4290	Extend the definition of a class to include identity relation.
class _I
Definition	df-eprel 4291*	Define the epsilon relation. Similar to Definition 6.22 of [TakeutiZaring] p. 30. The epsilon relation and set membership are the same, that is, ( A _E B <-> A e. B ) when B is a set by epelg 4292. Thus, 5 _E { 1 , 5 }. (Contributed by NM, 13-Aug-1995.)
|- _E = { <. x , y >. | x e. y }
Theorem	epelg 4292	The epsilon relation and membership are the same. General version of epel 4294. (Contributed by Scott Fenton, 27-Mar-2011.) (Revised by Mario Carneiro, 28-Apr-2015.)
|- ( B e. V -> ( A _E B <-> A e. B ) )
Theorem	epelc 4293	The epsilon relationship and the membership relation are the same. (Contributed by Scott Fenton, 11-Apr-2012.)
|- B e. _V   =>    |- ( A _E B <-> A e. B )
Theorem	epel 4294	The epsilon relation and the membership relation are the same. (Contributed by NM, 13-Aug-1995.)
|- ( x _E y <-> x e. y )
Definition	df-id 4295*	Define the identity relation. Definition 9.15 of [Quine] p. 64. For example, 5 _I 5 and -. 4 _I 5. (Contributed by NM, 13-Aug-1995.)
|- _I = { <. x , y >. | x = y }
2.3.8  Partial and total orderings We have not yet defined relations (df-rel 4635), but here we introduce a few related notions we will use to develop ordinals. The class variable R is no different from other class variables, but it reminds us that typically it represents what we will later call a "relation".
Syntax	wpo 4296	Extend wff notation to include the strict partial ordering predicate. Read: ' R is a partial order on A.'
wff R Po A
Syntax	wor 4297	Extend wff notation to include the strict linear ordering predicate. Read: ' R orders A.'
wff R Or A
Definition	df-po 4298*	Define the strict partial order predicate. Definition of [Enderton] p. 168. The expression R Po A means R is a partial order on A. (Contributed by NM, 16-Mar-1997.)
|- ( R Po A <-> A. x e. A A. y e. A A. z e. A ( -. x R x /\ ( ( x R y /\ y R z ) -> x R z ) ) )
Definition	df-iso 4299*	Define the strict linear order predicate. The expression R Or A is true if relationship R orders A. The property x R y -> ( x R z \/ z R y ) is called weak linearity by Proposition 11.2.3 of [HoTT], p. (varies). If we assumed excluded middle, it would be equivalent to trichotomy, x R y \/ x = y \/ y R x. (Contributed by NM, 21-Jan-1996.) (Revised by Jim Kingdon, 4-Oct-2018.)
|- ( R Or A <-> ( R Po A /\ A. x e. A A. y e. A A. z e. A ( x R y -> ( x R z \/ z R y ) ) ) )
Theorem	poss 4300	Subset theorem for the partial ordering predicate. (Contributed by NM, 27-Mar-1997.) (Proof shortened by Mario Carneiro, 18-Nov-2016.)
|- ( A C_ B -> ( R Po B -> R Po A ) )