P. 57 of Theorem List - Intuitionistic Logic Explorer

Theorem List for Intuitionistic Logic Explorer - 5601-5700   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	f1ff1 5601	If a function is one-to-one from A to B and is also a function from A to C, then it is a one-to-one function from A to C. (Contributed by BJ, 4-Jul-2022.)
|- ( ( F : A -1-1-> B /\ F : A --> C ) -> F : A -1-1-> C )
Theorem	f1ssres 5602	A function that is one-to-one is also one-to-one on any subclass of its domain. (Contributed by Mario Carneiro, 17-Jan-2015.)
|- ( ( F : A -1-1-> B /\ C C_ A ) -> ( F |` C ) : C -1-1-> B )
Theorem	f1resf1 5603	The restriction of an injective function is injective. (Contributed by AV, 28-Jun-2022.)
|- ( ( ( F : A -1-1-> B /\ C C_ A ) /\ ( F |` C ) : C --> D ) -> ( F |` C ) : C -1-1-> D )
Theorem	f1cnvcnv 5604	Two ways to express that a set A (not necessarily a function) is one-to-one. Each side is equivalent to Definition 6.4(3) of [TakeutiZaring] p. 24, who use the notation "Un2 (A)" for one-to-one. We do not introduce a separate notation since we rarely use it. (Contributed by NM, 13-Aug-2004.)
|- ( `' `' A : dom A -1-1-> _V <-> ( Fun `' A /\ Fun `' `' A ) )
Theorem	f1co 5605	Composition of one-to-one functions. Exercise 30 of [TakeutiZaring] p. 25. (Contributed by NM, 28-May-1998.)
|- ( ( F : B -1-1-> C /\ G : A -1-1-> B ) -> ( F o. G ) : A -1-1-> C )
Theorem	foeq1 5606	Equality theorem for onto functions. (Contributed by NM, 1-Aug-1994.)
|- ( F = G -> ( F : A -onto-> B <-> G : A -onto-> B ) )
Theorem	foeq2 5607	Equality theorem for onto functions. (Contributed by NM, 1-Aug-1994.)
|- ( A = B -> ( F : A -onto-> C <-> F : B -onto-> C ) )
Theorem	foeq3 5608	Equality theorem for onto functions. (Contributed by NM, 1-Aug-1994.)
|- ( A = B -> ( F : C -onto-> A <-> F : C -onto-> B ) )
Theorem	nffo 5609	Bound-variable hypothesis builder for an onto function. (Contributed by NM, 16-May-2004.)
|- F/_ x F   &    |- F/_ x A   &    |- F/_ x B   =>    |- F/ x F : A -onto-> B
Theorem	fof 5610	An onto mapping is a mapping. (Contributed by NM, 3-Aug-1994.)
|- ( F : A -onto-> B -> F : A --> B )
Theorem	fofun 5611	An onto mapping is a function. (Contributed by NM, 29-Mar-2008.)
|- ( F : A -onto-> B -> Fun F )
Theorem	fofn 5612	An onto mapping is a function on its domain. (Contributed by NM, 16-Dec-2008.)
|- ( F : A -onto-> B -> F Fn A )
Theorem	forn 5613	The codomain of an onto function is its range. (Contributed by NM, 3-Aug-1994.)
|- ( F : A -onto-> B -> ran F = B )
Theorem	dffo2 5614	Alternate definition of an onto function. (Contributed by NM, 22-Mar-2006.)
|- ( F : A -onto-> B <-> ( F : A --> B /\ ran F = B ) )
Theorem	foima 5615	The image of the domain of an onto function. (Contributed by NM, 29-Nov-2002.)
|- ( F : A -onto-> B -> ( F " A ) = B )
Theorem	dffn4 5616	A function maps onto its range. (Contributed by NM, 10-May-1998.)
|- ( F Fn A <-> F : A -onto-> ran F )
Theorem	funforn 5617	A function maps its domain onto its range. (Contributed by NM, 23-Jul-2004.)
|- ( Fun A <-> A : dom A -onto-> ran A )
Theorem	fodmrnu 5618	An onto function has unique domain and range. (Contributed by NM, 5-Nov-2006.)
|- ( ( F : A -onto-> B /\ F : C -onto-> D ) -> ( A = C /\ B = D ) )
Theorem	fimadmfo 5619	A function is a function onto the image of its domain. (Contributed by AV, 1-Dec-2022.)
|- ( F : A --> B -> F : A -onto-> ( F " A ) )
Theorem	fores 5620	Restriction of a function. (Contributed by NM, 4-Mar-1997.)
|- ( ( Fun F /\ A C_ dom F ) -> ( F |` A ) : A -onto-> ( F " A ) )
Theorem	foco 5621	Composition of onto functions. (Contributed by NM, 22-Mar-2006.)
|- ( ( F : B -onto-> C /\ G : A -onto-> B ) -> ( F o. G ) : A -onto-> C )
Theorem	f1oeq1 5622	Equality theorem for one-to-one onto functions. (Contributed by NM, 10-Feb-1997.)
|- ( F = G -> ( F : A -1-1-onto-> B <-> G : A -1-1-onto-> B ) )
Theorem	f1oeq2 5623	Equality theorem for one-to-one onto functions. (Contributed by NM, 10-Feb-1997.)
|- ( A = B -> ( F : A -1-1-onto-> C <-> F : B -1-1-onto-> C ) )
Theorem	f1oeq3 5624	Equality theorem for one-to-one onto functions. (Contributed by NM, 10-Feb-1997.)
|- ( A = B -> ( F : C -1-1-onto-> A <-> F : C -1-1-onto-> B ) )
Theorem	f1oeq23 5625	Equality theorem for one-to-one onto functions. (Contributed by FL, 14-Jul-2012.)
|- ( ( A = B /\ C = D ) -> ( F : A -1-1-onto-> C <-> F : B -1-1-onto-> D ) )
Theorem	f1eq123d 5626	Equality deduction for one-to-one functions. (Contributed by Mario Carneiro, 27-Jan-2017.)
|- ( ph -> F = G )   &    |- ( ph -> A = B )   &    |- ( ph -> C = D )   =>    |- ( ph -> ( F : A -1-1-> C <-> G : B -1-1-> D ) )
Theorem	foeq123d 5627	Equality deduction for onto functions. (Contributed by Mario Carneiro, 27-Jan-2017.)
|- ( ph -> F = G )   &    |- ( ph -> A = B )   &    |- ( ph -> C = D )   =>    |- ( ph -> ( F : A -onto-> C <-> G : B -onto-> D ) )
Theorem	f1oeq123d 5628	Equality deduction for one-to-one onto functions. (Contributed by Mario Carneiro, 27-Jan-2017.)
|- ( ph -> F = G )   &    |- ( ph -> A = B )   &    |- ( ph -> C = D )   =>    |- ( ph -> ( F : A -1-1-onto-> C <-> G : B -1-1-onto-> D ) )
Theorem	f1oeq1d 5629	Equality deduction for one-to-one onto functions. (Contributed by Glauco Siliprandi, 17-Aug-2020.)
|- ( ph -> F = G )   =>    |- ( ph -> ( F : A -1-1-onto-> B <-> G : A -1-1-onto-> B ) )
Theorem	f1oeq2d 5630	Equality deduction for one-to-one onto functions. (Contributed by Glauco Siliprandi, 17-Aug-2020.)
|- ( ph -> A = B )   =>    |- ( ph -> ( F : A -1-1-onto-> C <-> F : B -1-1-onto-> C ) )
Theorem	f1oeq3d 5631	Equality deduction for one-to-one onto functions. (Contributed by Glauco Siliprandi, 17-Aug-2020.)
|- ( ph -> A = B )   =>    |- ( ph -> ( F : C -1-1-onto-> A <-> F : C -1-1-onto-> B ) )
Theorem	nff1o 5632	Bound-variable hypothesis builder for a one-to-one onto function. (Contributed by NM, 16-May-2004.)
|- F/_ x F   &    |- F/_ x A   &    |- F/_ x B   =>    |- F/ x F : A -1-1-onto-> B
Theorem	f1of1 5633	A one-to-one onto mapping is a one-to-one mapping. (Contributed by NM, 12-Dec-2003.)
|- ( F : A -1-1-onto-> B -> F : A -1-1-> B )
Theorem	f1of 5634	A one-to-one onto mapping is a mapping. (Contributed by NM, 12-Dec-2003.)
|- ( F : A -1-1-onto-> B -> F : A --> B )
Theorem	f1ofn 5635	A one-to-one onto mapping is function on its domain. (Contributed by NM, 12-Dec-2003.)
|- ( F : A -1-1-onto-> B -> F Fn A )
Theorem	f1ofun 5636	A one-to-one onto mapping is a function. (Contributed by NM, 12-Dec-2003.)
|- ( F : A -1-1-onto-> B -> Fun F )
Theorem	f1orel 5637	A one-to-one onto mapping is a relation. (Contributed by NM, 13-Dec-2003.)
|- ( F : A -1-1-onto-> B -> Rel F )
Theorem	f1odm 5638	The domain of a one-to-one onto mapping. (Contributed by NM, 8-Mar-2014.)
|- ( F : A -1-1-onto-> B -> dom F = A )
Theorem	dff1o2 5639	Alternate definition of one-to-one onto function. (Contributed by NM, 10-Feb-1997.) (Proof shortened by Andrew Salmon, 22-Oct-2011.)
|- ( F : A -1-1-onto-> B <-> ( F Fn A /\ Fun `' F /\ ran F = B ) )
Theorem	dff1o3 5640	Alternate definition of one-to-one onto function. (Contributed by NM, 25-Mar-1998.) (Proof shortened by Andrew Salmon, 22-Oct-2011.)
|- ( F : A -1-1-onto-> B <-> ( F : A -onto-> B /\ Fun `' F ) )
Theorem	f1ofo 5641	A one-to-one onto function is an onto function. (Contributed by NM, 28-Apr-2004.)
|- ( F : A -1-1-onto-> B -> F : A -onto-> B )
Theorem	dff1o4 5642	Alternate definition of one-to-one onto function. (Contributed by NM, 25-Mar-1998.) (Proof shortened by Andrew Salmon, 22-Oct-2011.)
|- ( F : A -1-1-onto-> B <-> ( F Fn A /\ `' F Fn B ) )
Theorem	dff1o5 5643	Alternate definition of one-to-one onto function. (Contributed by NM, 10-Dec-2003.) (Proof shortened by Andrew Salmon, 22-Oct-2011.)
|- ( F : A -1-1-onto-> B <-> ( F : A -1-1-> B /\ ran F = B ) )
Theorem	f1orn 5644	A one-to-one function maps onto its range. (Contributed by NM, 13-Aug-2004.)
|- ( F : A -1-1-onto-> ran F <-> ( F Fn A /\ Fun `' F ) )
Theorem	f1f1orn 5645	A one-to-one function maps one-to-one onto its range. (Contributed by NM, 4-Sep-2004.)
|- ( F : A -1-1-> B -> F : A -1-1-onto-> ran F )
Theorem	f1oabexg 5646*	The class of all 1-1-onto functions mapping one set to another is a set. (Contributed by Paul Chapman, 25-Feb-2008.)
|- F = { f | ( f : A -1-1-onto-> B /\ ph ) }   =>    |- ( ( A e. C /\ B e. D ) -> F e. _V )
Theorem	f1ocnv 5647	The converse of a one-to-one onto function is also one-to-one onto. (Contributed by NM, 11-Feb-1997.) (Proof shortened by Andrew Salmon, 22-Oct-2011.)
|- ( F : A -1-1-onto-> B -> `' F : B -1-1-onto-> A )
Theorem	f1ocnvb 5648	A relation is a one-to-one onto function iff its converse is a one-to-one onto function with domain and codomain/range interchanged. (Contributed by NM, 8-Dec-2003.)
|- ( Rel F -> ( F : A -1-1-onto-> B <-> `' F : B -1-1-onto-> A ) )
Theorem	f1ores 5649	The restriction of a one-to-one function maps one-to-one onto the image. (Contributed by NM, 25-Mar-1998.)
|- ( ( F : A -1-1-> B /\ C C_ A ) -> ( F |` C ) : C -1-1-onto-> ( F " C ) )
Theorem	f1orescnv 5650	The converse of a one-to-one-onto restricted function. (Contributed by Paul Chapman, 21-Apr-2008.)
|- ( ( Fun `' F /\ ( F |` R ) : R -1-1-onto-> P ) -> ( `' F |` P ) : P -1-1-onto-> R )
Theorem	f1imacnv 5651	Preimage of an image. (Contributed by NM, 30-Sep-2004.)
|- ( ( F : A -1-1-> B /\ C C_ A ) -> ( `' F " ( F " C ) ) = C )
Theorem	foimacnv 5652	A reverse version of f1imacnv 5651. (Contributed by Jeff Hankins, 16-Jul-2009.)
|- ( ( F : A -onto-> B /\ C C_ B ) -> ( F " ( `' F " C ) ) = C )
Theorem	foun 5653	The union of two onto functions with disjoint domains is an onto function. (Contributed by Mario Carneiro, 22-Jun-2016.)
|- ( ( ( F : A -onto-> B /\ G : C -onto-> D ) /\ ( A i^i C ) = (/) ) -> ( F u. G ) : ( A u. C ) -onto-> ( B u. D ) )
Theorem	f1oun 5654	The union of two one-to-one onto functions with disjoint domains and ranges. (Contributed by NM, 26-Mar-1998.)
|- ( ( ( F : A -1-1-onto-> B /\ G : C -1-1-onto-> D ) /\ ( ( A i^i C ) = (/) /\ ( B i^i D ) = (/) ) ) -> ( F u. G ) : ( A u. C ) -1-1-onto-> ( B u. D ) )
Theorem	fun11iun 5655*	The union of a chain (with respect to inclusion) of one-to-one functions is a one-to-one function. (Contributed by Mario Carneiro, 20-May-2013.) (Revised by Mario Carneiro, 24-Jun-2015.)
|- ( x = y -> B = C )   &    |- B e. _V   =>    |- ( A. x e. A ( B : D -1-1-> S /\ A. y e. A ( B C_ C \/ C C_ B ) ) -> U_ x e. A B : U_ x e. A D -1-1-> S )
Theorem	resdif 5656	The restriction of a one-to-one onto function to a difference maps onto the difference of the images. (Contributed by Paul Chapman, 11-Apr-2009.)
|- ( ( Fun `' F /\ ( F |` A ) : A -onto-> C /\ ( F |` B ) : B -onto-> D ) -> ( F |` ( A \ B ) ) : ( A \ B ) -1-1-onto-> ( C \ D ) )
Theorem	f1oco 5657	Composition of one-to-one onto functions. (Contributed by NM, 19-Mar-1998.)
|- ( ( F : B -1-1-onto-> C /\ G : A -1-1-onto-> B ) -> ( F o. G ) : A -1-1-onto-> C )
Theorem	f1cnv 5658	The converse of an injective function is bijective. (Contributed by FL, 11-Nov-2011.)
|- ( F : A -1-1-> B -> `' F : ran F -1-1-onto-> A )
Theorem	funcocnv2 5659	Composition with the converse. (Contributed by Jeff Madsen, 2-Sep-2009.)
|- ( Fun F -> ( F o. `' F ) = ( _I |` ran F ) )
Theorem	fococnv2 5660	The composition of an onto function and its converse. (Contributed by Stefan O'Rear, 12-Feb-2015.)
|- ( F : A -onto-> B -> ( F o. `' F ) = ( _I |` B ) )
Theorem	f1ococnv2 5661	The composition of a one-to-one onto function and its converse equals the identity relation restricted to the function's range. (Contributed by NM, 13-Dec-2003.) (Proof shortened by Stefan O'Rear, 12-Feb-2015.)
|- ( F : A -1-1-onto-> B -> ( F o. `' F ) = ( _I |` B ) )
Theorem	f1cocnv2 5662	Composition of an injective function with its converse. (Contributed by FL, 11-Nov-2011.)
|- ( F : A -1-1-> B -> ( F o. `' F ) = ( _I |` ran F ) )
Theorem	f1ococnv1 5663	The composition of a one-to-one onto function's converse and itself equals the identity relation restricted to the function's domain. (Contributed by NM, 13-Dec-2003.)
|- ( F : A -1-1-onto-> B -> ( `' F o. F ) = ( _I |` A ) )
Theorem	f1cocnv1 5664	Composition of an injective function with its converse. (Contributed by FL, 11-Nov-2011.)
|- ( F : A -1-1-> B -> ( `' F o. F ) = ( _I |` A ) )
Theorem	funcoeqres 5665	Express a constraint on a composition as a constraint on the composand. (Contributed by Stefan O'Rear, 7-Mar-2015.)
|- ( ( Fun G /\ ( F o. G ) = H ) -> ( F |` ran G ) = ( H o. `' G ) )
Theorem	f1ssf1 5666	A subset of an injective function is injective. (Contributed by AV, 20-Nov-2020.)
|- ( ( Fun F /\ Fun `' F /\ G C_ F ) -> Fun `' G )
Theorem	ffoss 5667*	Relationship between a mapping and an onto mapping. Figure 38 of [Enderton] p. 145. (Contributed by NM, 10-May-1998.)
|- F e. _V   =>    |- ( F : A --> B <-> E. x ( F : A -onto-> x /\ x C_ B ) )
Theorem	f11o 5668*	Relationship between one-to-one and one-to-one onto function. (Contributed by NM, 4-Apr-1998.)
|- F e. _V   =>    |- ( F : A -1-1-> B <-> E. x ( F : A -1-1-onto-> x /\ x C_ B ) )
Theorem	f10 5669	The empty set maps one-to-one into any class. (Contributed by NM, 7-Apr-1998.)
|- (/) : (/) -1-1-> A
Theorem	f10d 5670	The empty set maps one-to-one into any class, deduction version. (Contributed by AV, 25-Nov-2020.)
|- ( ph -> F = (/) )   =>    |- ( ph -> F : dom F -1-1-> A )
Theorem	f1o00 5671	One-to-one onto mapping of the empty set. (Contributed by NM, 15-Apr-1998.)
|- ( F : (/) -1-1-onto-> A <-> ( F = (/) /\ A = (/) ) )
Theorem	fo00 5672	Onto mapping of the empty set. (Contributed by NM, 22-Mar-2006.)
|- ( F : (/) -onto-> A <-> ( F = (/) /\ A = (/) ) )
Theorem	f1o0 5673	One-to-one onto mapping of the empty set. (Contributed by NM, 10-Sep-2004.)
|- (/) : (/) -1-1-onto-> (/)
Theorem	f1oi 5674	A restriction of the identity relation is a one-to-one onto function. (Contributed by NM, 30-Apr-1998.) (Proof shortened by Andrew Salmon, 22-Oct-2011.)
|- ( _I |` A ) : A -1-1-onto-> A
Theorem	f1ovi 5675	The identity relation is a one-to-one onto function on the universe. (Contributed by NM, 16-May-2004.)
|- _I : _V -1-1-onto-> _V
Theorem	f1osn 5676	A singleton of an ordered pair is one-to-one onto function. (Contributed by NM, 18-May-1998.) (Proof shortened by Andrew Salmon, 22-Oct-2011.)
|- A e. _V   &    |- B e. _V   =>    |- { <. A , B >. } : { A } -1-1-onto-> { B }
Theorem	f1osng 5677	A singleton of an ordered pair is one-to-one onto function. (Contributed by Mario Carneiro, 12-Jan-2013.)
|- ( ( A e. V /\ B e. W ) -> { <. A , B >. } : { A } -1-1-onto-> { B } )
Theorem	f1sng 5678	A singleton of an ordered pair is a one-to-one function. (Contributed by AV, 17-Apr-2021.)
|- ( ( A e. V /\ B e. W ) -> { <. A , B >. } : { A } -1-1-> W )
Theorem	fsnd 5679	A singleton of an ordered pair is a function. (Contributed by AV, 17-Apr-2021.)
|- ( ph -> A e. V )   &    |- ( ph -> B e. W )   =>    |- ( ph -> { <. A , B >. } : { A } --> W )
Theorem	f1oprg 5680	An unordered pair of ordered pairs with different elements is a one-to-one onto function. (Contributed by Alexander van der Vekens, 14-Aug-2017.)
|- ( ( ( A e. V /\ B e. W ) /\ ( C e. X /\ D e. Y ) ) -> ( ( A =/= C /\ B =/= D ) -> { <. A , B >. , <. C , D >. } : { A , C } -1-1-onto-> { B , D } ) )
Theorem	tz6.12-2 5681*	Function value when F is not a function. Theorem 6.12(2) of [TakeutiZaring] p. 27. (Contributed by NM, 30-Apr-2004.) (Proof shortened by Mario Carneiro, 31-Aug-2015.)
|- ( -. E! x A F x -> ( F ` A ) = (/) )
Theorem	fveu 5682*	The value of a function at a unique point. (Contributed by Scott Fenton, 6-Oct-2017.)
|- ( E! x A F x -> ( F ` A ) = U. { x | A F x } )
Theorem	brprcneu 5683*	If A is a proper class and F is any class, then there is no unique set which is related to A through the binary relation F. (Contributed by Scott Fenton, 7-Oct-2017.)
|- ( -. A e. _V -> -. E! x A F x )
Theorem	fvprc 5684	A function's value at a proper class is the empty set. (Contributed by NM, 20-May-1998.)
|- ( -. A e. _V -> ( F ` A ) = (/) )
Theorem	fv2 5685*	Alternate definition of function value. Definition 10.11 of [Quine] p. 68. (Contributed by NM, 30-Apr-2004.) (Proof shortened by Andrew Salmon, 17-Sep-2011.) (Revised by Mario Carneiro, 31-Aug-2015.)
|- ( F ` A ) = U. { x | A. y ( A F y <-> y = x ) }
Theorem	dffv3g 5686*	A definition of function value in terms of iota. (Contributed by Jim Kingdon, 29-Dec-2018.)
|- ( A e. V -> ( F ` A ) = ( iota x x e. ( F " { A } ) ) )
Theorem	dffv4g 5687*	The previous definition of function value, from before the iota operator was introduced. Although based on the idea embodied by Definition 10.2 of [Quine] p. 65 (see args 5151), this definition apparently does not appear in the literature. (Contributed by NM, 1-Aug-1994.)
|- ( A e. V -> ( F ` A ) = U. { x | ( F " { A } ) = { x } } )
Theorem	elfv 5688*	Membership in a function value. (Contributed by NM, 30-Apr-2004.)
|- ( A e. ( F ` B ) <-> E. x ( A e. x /\ A. y ( B F y <-> y = x ) ) )
Theorem	fveq1 5689	Equality theorem for function value. (Contributed by NM, 29-Dec-1996.)
|- ( F = G -> ( F ` A ) = ( G ` A ) )
Theorem	fveq2 5690	Equality theorem for function value. (Contributed by NM, 29-Dec-1996.)
|- ( A = B -> ( F ` A ) = ( F ` B ) )
Theorem	fveq1i 5691	Equality inference for function value. (Contributed by NM, 2-Sep-2003.)
|- F = G   =>    |- ( F ` A ) = ( G ` A )
Theorem	fveq1d 5692	Equality deduction for function value. (Contributed by NM, 2-Sep-2003.)
|- ( ph -> F = G )   =>    |- ( ph -> ( F ` A ) = ( G ` A ) )
Theorem	fveq2i 5693	Equality inference for function value. (Contributed by NM, 28-Jul-1999.)
|- A = B   =>    |- ( F ` A ) = ( F ` B )
Theorem	fveq2d 5694	Equality deduction for function value. (Contributed by NM, 29-May-1999.)
|- ( ph -> A = B )   =>    |- ( ph -> ( F ` A ) = ( F ` B ) )
Theorem	2fveq3 5695	Equality theorem for nested function values. (Contributed by AV, 14-Aug-2022.)
|- ( A = B -> ( F ` ( G ` A ) ) = ( F ` ( G ` B ) ) )
Theorem	fveq12i 5696	Equality deduction for function value. (Contributed by FL, 27-Jun-2014.)
|- F = G   &    |- A = B   =>    |- ( F ` A ) = ( G ` B )
Theorem	fveq12d 5697	Equality deduction for function value. (Contributed by FL, 22-Dec-2008.)
|- ( ph -> F = G )   &    |- ( ph -> A = B )   =>    |- ( ph -> ( F ` A ) = ( G ` B ) )
Theorem	fveqeq2d 5698	Equality deduction for function value. (Contributed by BJ, 30-Aug-2022.)
|- ( ph -> A = B )   =>    |- ( ph -> ( ( F ` A ) = C <-> ( F ` B ) = C ) )
Theorem	fveqeq2 5699	Equality deduction for function value. (Contributed by BJ, 31-Aug-2022.)
|- ( A = B -> ( ( F ` A ) = C <-> ( F ` B ) = C ) )
Theorem	nffv 5700	Bound-variable hypothesis builder for function value. (Contributed by NM, 14-Nov-1995.) (Revised by Mario Carneiro, 15-Oct-2016.)
|- F/_ x F   &    |- F/_ x A   =>    |- F/_ x ( F ` A )