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**Theorem List for Intuitionistic Logic Explorer -** 16801-16898 *Has distinct variable group(s)
Type	Label	Description
Statement

Theorem	peano4nninf 16801*	The successor function on ℕ_∞ is one to one. Half of Lemma 3.4 of [PradicBrown2022], p. 5. (Contributed by Jim Kingdon, 31-Jul-2022.)
ℕ_∞ ℕ_∞ℕ_∞

Theorem	peano3nninf 16802*	The successor function on ℕ_∞ is never zero. Half of Lemma 3.4 of [PradicBrown2022], p. 5. (Contributed by Jim Kingdon, 1-Aug-2022.)
ℕ_∞ ℕ_∞

Theorem	nninfalllem1 16803*	Lemma for nninfall 16804. (Contributed by Jim Kingdon, 1-Aug-2022.)
ℕ_∞ ℕ_∞

Theorem	nninfall 16804*	Given a decidable predicate on ℕ_∞, showing it holds for natural numbers and the point at infinity suffices to show it holds everywhere. The sense in which is a decidable predicate is that it assigns a value of either or (which can be thought of as false and true) to every element of ℕ_∞. Lemma 3.5 of [PradicBrown2022], p. 5. (Contributed by Jim Kingdon, 1-Aug-2022.)
ℕ_∞ ℕ_∞

Theorem	nninfsellemdc 16805*	Lemma for nninfself 16808. Showing that the selection function is well defined. (Contributed by Jim Kingdon, 8-Aug-2022.)
ℕ_∞ $/\$ DECID

Theorem	nninfsellemcl 16806*	Lemma for nninfself 16808. (Contributed by Jim Kingdon, 8-Aug-2022.)
ℕ_∞ $/\$

Theorem	nninfsellemsuc 16807*	Lemma for nninfself 16808. (Contributed by Jim Kingdon, 6-Aug-2022.)
ℕ_∞ $/\$

Theorem	nninfself 16808*	Domain and range of the selection function for ℕ_∞. (Contributed by Jim Kingdon, 6-Aug-2022.)
ℕ_∞ ℕ_∞ℕ_∞

Theorem	nninfsellemeq 16809*	Lemma for nninfsel 16812. (Contributed by Jim Kingdon, 9-Aug-2022.)
ℕ_∞ ℕ_∞

Theorem	nninfsellemqall 16810*	Lemma for nninfsel 16812. (Contributed by Jim Kingdon, 9-Aug-2022.)
ℕ_∞ ℕ_∞

Theorem	nninfsellemeqinf 16811*	Lemma for nninfsel 16812. (Contributed by Jim Kingdon, 9-Aug-2022.)
ℕ_∞ ℕ_∞

Theorem	nninfsel 16812*	is a selection function for ℕ_∞. Theorem 3.6 of [PradicBrown2022], p. 5. (Contributed by Jim Kingdon, 9-Aug-2022.)
ℕ_∞ ℕ_∞ ℕ_∞

Theorem	nninfomnilem 16813*	Lemma for nninfomni 16814. (Contributed by Jim Kingdon, 10-Aug-2022.)
ℕ_∞ ℕ_∞ Omni

Theorem	nninfomni 16814	ℕ_∞ is omniscient. Corollary 3.7 of [PradicBrown2022], p. 5. (Contributed by Jim Kingdon, 10-Aug-2022.)
ℕ_∞ Omni

Theorem	nninffeq 16815*	Equality of two functions on ℕ_∞ which agree at every integer and at the point at infinity. From an online post by Martin Escardo. Remark: the last two hypotheses can be grouped into one, . (Contributed by Jim Kingdon, 4-Aug-2023.)
ℕ_∞ ℕ_∞

Theorem	nnnninfen 16816	Equinumerosity of the natural numbers and ℕ_∞ is equivalent to the Limited Principle of Omniscience (LPO). Remark in Section 1.1 of [Pradic2025], p. 2. (Contributed by Jim Kingdon, 8-Jul-2025.)
ℕ_∞ Omni

Theorem	nnnninfex 16817*	If an element of ℕ_∞ has a value of zero somewhere, then it is the mapping of a natural number. (Contributed by Jim Kingdon, 4-Aug-2022.)
ℕ_∞

Theorem	nninfnfiinf 16818*	An element of ℕ_∞ which is not finite is infinite. (Contributed by Jim Kingdon, 30-Nov-2025.)
ℕ_∞ $/\$

14.4.5 Schroeder-Bernstein Theorem

Theorem	exmidsbthrlem 16819*	Lemma for exmidsbthr 16820. (Contributed by Jim Kingdon, 11-Aug-2022.)
ℕ_∞ $/\$ EXMID

Theorem	exmidsbthr 16820*	The Schroeder-Bernstein Theorem implies excluded middle. Theorem 1 of [PradicBrown2022], p. 1. (Contributed by Jim Kingdon, 11-Aug-2022.)
$/\$ EXMID

Theorem	exmidsbth 16821*	The Schroeder-Bernstein Theorem is equivalent to excluded middle. This is Metamath 100 proof #25. The forward direction (isbth 7239) is the proof of the Schroeder-Bernstein Theorem from the Metamath Proof Explorer database (in which excluded middle holds), but adapted to use EXMID as an antecedent rather than being unconditionally true, as in the non-intuitionistic proof at https://us.metamath.org/mpeuni/sbth.html 7239. The reverse direction (exmidsbthr 16820) is the one which establishes that Schroeder-Bernstein implies excluded middle. This resolves the question of whether we will be able to prove Schroeder-Bernstein from our axioms in the negative. (Contributed by Jim Kingdon, 13-Aug-2022.)
EXMID $/\$

Theorem	sbthomlem 16822	Lemma for sbthom 16823. (Contributed by Mario Carneiro and Jim Kingdon, 13-Jul-2023.)
Omni ⊔ $\/$

Theorem	sbthom 16823	Schroeder-Bernstein is not possible even for . We know by exmidsbth 16821 that full Schroeder-Bernstein will not be provable but what about the case where one of the sets is ? That case plus the Limited Principle of Omniscience (LPO) implies excluded middle, so we will not be able to prove it. (Contributed by Mario Carneiro and Jim Kingdon, 10-Jul-2023.)
$/\$ $/\$ Omni EXMID

14.4.6 Real and complex numbers

Theorem	qdencn 16824*	The set of complex numbers whose real and imaginary parts are rational is dense in the complex plane. This is a two dimensional analogue to qdenre 11891 (and also would hold for with the usual metric; this is not about complex numbers in particular). (Contributed by Jim Kingdon, 18-Oct-2021.)
$/\$ $/\$

Theorem	refeq 16825*	Equality of two real functions which agree at negative numbers, positive numbers, and zero. This holds even without real trichotomy. From an online post by Martin Escardo. (Contributed by Jim Kingdon, 9-Jul-2023.)


Theorem	repiecelem 16826*	Lemma for repiecele0 16827, repiecege0 16828, and repiecef 16829. The function is defined everywhere. (Contributed by Jim Kingdon, 27-Apr-2026.)
inf $/\$ inf

Theorem	repiecele0 16827*	Piecewise definition on the reals agrees with the nonpositive part of the definition. See repiecef 16829 for more on this construction. (Contributed by Jim Kingdon, 27-Apr-2026.)
inf $/\$ $/\$

Theorem	repiecege0 16828*	Piecewise definition on the reals agrees with the nonnegative part of the definition. See repiecef 16829 for more on this construction. (Contributed by Jim Kingdon, 27-Apr-2026.)
inf $/\$ $/\$

Theorem	repiecef 16829*	Piecewise definition on the reals yields a function. The function agrees with and on their respective parts of the real line; see repiecele0 16827 and repiecege0 16828. From an online post by James E Hanson. The construction was published in Martín Hötzel Escardó, "Effective and sequential definition by cases on the reals via infinite signed-digit numerals", Electronic Notes in Theoretical Computer Science 10 (1998), page 2, https://martinescardo.github.io/papers/lexnew.pdf. 16828 (Contributed by Jim Kingdon, 27-Apr-2026.)
inf

Theorem	triap 16830	Two ways of stating real number trichotomy. See also cndcap 16861 which is similar but for complex number apartness. (Contributed by Jim Kingdon, 23-Aug-2023.)
$/\$ $\/$ $\/$ DECID #

Theorem	isomninnlem 16831*	Lemma for isomninn 16832. The result, with a hypothesis to provide a convenient notation. (Contributed by Jim Kingdon, 30-Aug-2023.)
frec Omni $\/$

Theorem	isomninn 16832*	Omniscience stated in terms of natural numbers. Similar to isomnimap 7430 but it will sometimes be more convenient to use and rather than and . (Contributed by Jim Kingdon, 30-Aug-2023.)
Omni $\/$

Theorem	cvgcmp2nlemabs 16833*	Lemma for cvgcmp2n 16834. The partial sums get closer to each other as we go further out. The proof proceeds by rewriting as the sum of and a term which gets smaller as gets large. (Contributed by Jim Kingdon, 25-Aug-2023.)
$/\$ $/\$ $/\$

Theorem	cvgcmp2n 16834*	A comparison test for convergence of a real infinite series. (Contributed by Jim Kingdon, 25-Aug-2023.)
$/\$ $/\$ $/\$

Theorem	iooref1o 16835	A one-to-one mapping from the real numbers onto the open unit interval. (Contributed by Jim Kingdon, 27-Jun-2024.)


Theorem	iooreen 16836	An open interval is equinumerous to the real numbers. (Contributed by Jim Kingdon, 27-Jun-2024.)


14.4.7 Analytic omniscience principles Omniscience principles refer to several propositions, most of them weaker than full excluded middle, which do not follow from the axioms of IZF set theory. They are: (0) the Principle of Omniscience (PO), which is another name for excluded middle (see exmidomni 7435), (1) the Limited Principle of Omniscience (LPO) is Omni (see df-omni 7428), (2) the Weak Limited Principle of Omniscience (WLPO) is WOmni (see df-womni 7457), (3) Markov's Principle (MP) is Markov (see df-markov 7445), (4) the Lesser Limited Principle of Omniscience (LLPO) is not yet defined in iset.mm. They also have analytic counterparts each of which follows from the corresponding omniscience principle: (1) Analytic LPO is real number trichotomy, $\/$ $\/$ (see trilpo 16844), (2) Analytic WLPO is decidability of real number equality, DECID (see redcwlpo 16857), (3) Analytic MP is # (see neapmkv 16871), (4) Analytic LLPO is real number dichotomy, $\/$ (most relevant current theorem is maxclpr 11911).

Theorem	trilpolemclim 16837*	Lemma for trilpo 16844. Convergence of the series. (Contributed by Jim Kingdon, 24-Aug-2023.)


Theorem	trilpolemcl 16838*	Lemma for trilpo 16844. The sum exists. (Contributed by Jim Kingdon, 23-Aug-2023.)


Theorem	trilpolemisumle 16839*	Lemma for trilpo 16844. An upper bound for the sum of the digits beyond a certain point. (Contributed by Jim Kingdon, 28-Aug-2023.)


Theorem	trilpolemgt1 16840*	Lemma for trilpo 16844. The case. (Contributed by Jim Kingdon, 23-Aug-2023.)


Theorem	trilpolemeq1 16841*	Lemma for trilpo 16844. The case. This is proved by noting that if any is zero, then the infinite sum is less than one based on the term which is zero. We are using the fact that the sequence is decidable (in the sense that each element is either zero or one). (Contributed by Jim Kingdon, 23-Aug-2023.)


Theorem	trilpolemlt1 16842*	Lemma for trilpo 16844. The case. We can use the distance between and one (that is, ) to find a position in the sequence where terms after that point will not add up to as much as . By finomni 7433 we know the terms up to either contain a zero or are all one. But if they are all one that contradicts the way we constructed , so we know that the sequence contains a zero. (Contributed by Jim Kingdon, 23-Aug-2023.)


Theorem	trilpolemres 16843*	Lemma for trilpo 16844. The result. (Contributed by Jim Kingdon, 23-Aug-2023.)
$\/$ $\/$ $\/$

Theorem	trilpo 16844*	Real number trichotomy implies the Limited Principle of Omniscience (LPO). We expect that we'd need some form of countable choice to prove the converse. Here's the outline of the proof. Given an infinite sequence F of zeroes and ones, we need to show the sequence contains a zero or it is all ones. Construct a real number A whose representation in base two consists of a zero, a decimal point, and then the numbers of the sequence. Compare it with one using trichotomy. The three cases from trichotomy are trilpolemlt1 16842 (which means the sequence contains a zero), trilpolemeq1 16841 (which means the sequence is all ones), and trilpolemgt1 16840 (which is not possible). Equivalent ways to state real number trichotomy (sometimes called "analytic LPO") include decidability of real number apartness (see triap 16830) or that the real numbers are a discrete field (see trirec0 16845). LPO is known to not be provable in IZF (and most constructive foundations), so this theorem establishes that we will be unable to prove an analogue to qtri3or 10604 for real numbers. (Contributed by Jim Kingdon, 23-Aug-2023.)
$\/$ $\/$ Omni

Theorem	trirec0 16845*	Every real number having a reciprocal or equaling zero is equivalent to real number trichotomy. This is the key part of the definition of what is known as a discrete field, so "the real numbers are a discrete field" can be taken as an equivalent way to state real trichotomy (see further discussion at trilpo 16844). (Contributed by Jim Kingdon, 10-Jun-2024.)
$\/$ $\/$ $\/$

Theorem	trirec0xor 16846*	Version of trirec0 16845 with exclusive-or. The definition of a discrete field is sometimes stated in terms of exclusive-or but as proved here, this is equivalent to inclusive-or because the two disjuncts cannot be simultaneously true. (Contributed by Jim Kingdon, 10-Jun-2024.)
$\/$ $\/$ $\/_$

Theorem	apdifflemf 16847	Lemma for apdiff 16849. Being apart from the point halfway between and suffices for to be a different distance from and from . (Contributed by Jim Kingdon, 18-May-2024.)
# #

Theorem	apdifflemr 16848	Lemma for apdiff 16849. (Contributed by Jim Kingdon, 19-May-2024.)
# $/\$ # #

Theorem	apdiff 16849*	The irrationals (reals apart from any rational) are exactly those reals that are a different distance from every rational. (Contributed by Jim Kingdon, 17-May-2024.)
# #

Theorem	qdiff 16850*	The rationals are exactly those reals for which there exist two distinct rationals that are the same distance from the original number. Similar to apdiff 16849 but by stating the result positively we can completely sidestep the issue of not equal versus apart in the statement of the result. From an online post by Ingo Blechschmidt. (Contributed by Jim Kingdon, 24-Apr-2026.)
$/\$

Theorem	iswomninnlem 16851*	Lemma for iswomnimap 7459. The result, with a hypothesis for convenience. (Contributed by Jim Kingdon, 20-Jun-2024.)
frec WOmni DECID

Theorem	iswomninn 16852*	Weak omniscience stated in terms of natural numbers. Similar to iswomnimap 7459 but it will sometimes be more convenient to use and rather than and . (Contributed by Jim Kingdon, 20-Jun-2024.)
WOmni DECID

Theorem	iswomni0 16853*	Weak omniscience stated in terms of equality with . Like iswomninn 16852 but with zero in place of one. (Contributed by Jim Kingdon, 24-Jul-2024.)
WOmni DECID

Theorem	ismkvnnlem 16854*	Lemma for ismkvnn 16855. The result, with a hypothesis to give a name to an expression for convenience. (Contributed by Jim Kingdon, 25-Jun-2024.)
frec Markov

Theorem	ismkvnn 16855*	The predicate of being Markov stated in terms of set exponentiation. (Contributed by Jim Kingdon, 25-Jun-2024.)
Markov

Theorem	redcwlpolemeq1 16856*	Lemma for redcwlpo 16857. A biconditionalized version of trilpolemeq1 16841. (Contributed by Jim Kingdon, 21-Jun-2024.)


Theorem	redcwlpo 16857*	Decidability of real number equality implies the Weak Limited Principle of Omniscience (WLPO). We expect that we'd need some form of countable choice to prove the converse. Here's the outline of the proof. Given an infinite sequence F of zeroes and ones, we need to show the sequence is all ones or it is not. Construct a real number A whose representation in base two consists of a zero, a decimal point, and then the numbers of the sequence. This real number will equal one if and only if the sequence is all ones (redcwlpolemeq1 16856). Therefore decidability of real number equality would imply decidability of whether the sequence is all ones. Because of this theorem, decidability of real number equality is sometimes called "analytic WLPO". WLPO is known to not be provable in IZF (and most constructive foundations), so this theorem establishes that we will be unable to prove an analogue to qdceq 10608 for real numbers. (Contributed by Jim Kingdon, 20-Jun-2024.)
DECID WOmni

Theorem	tridceq 16858*	Real trichotomy implies decidability of real number equality. Or in other words, analytic LPO implies analytic WLPO (see trilpo 16844 and redcwlpo 16857). Thus, this is an analytic analogue to lpowlpo 7461. (Contributed by Jim Kingdon, 24-Jul-2024.)
$\/$ $\/$ DECID

Theorem	redc0 16859*	Two ways to express decidability of real number equality. (Contributed by Jim Kingdon, 23-Jul-2024.)
DECID DECID

Theorem	reap0 16860*	Real number trichotomy is equivalent to decidability of apartness from zero. (Contributed by Jim Kingdon, 27-Jul-2024.)
$\/$ $\/$ DECID #

Theorem	cndcap 16861*	Real number trichotomy is equivalent to decidability of complex number apartness. (Contributed by Jim Kingdon, 10-Apr-2025.)
$\/$ $\/$ DECID #

Theorem	trimul0or 16862*	Real number trichotomy implies that if a product is zero, one of its factors must be zero. (Contributed by Jim Kingdon, 27-May-2026.)
$\/$ $\/$ $\/$

Theorem	dceqnconst 16863*	Decidability of real number equality implies the existence of a certain non-constant function from real numbers to integers. Variation of Exercise 11.6(i) of [HoTT], p. (varies). See redcwlpo 16857 for more discussion of decidability of real number equality. (Contributed by BJ and Jim Kingdon, 24-Jun-2024.) (Revised by Jim Kingdon, 23-Jul-2024.)
DECID $/\$ $/\$

Theorem	dcapnconst 16864*	Decidability of real number apartness implies the existence of a certain non-constant function from real numbers to integers. Variation of Exercise 11.6(i) of [HoTT], p. (varies). See trilpo 16844 for more discussion of decidability of real number apartness. This is a weaker form of dceqnconst 16863 and in fact this theorem can be proved using dceqnconst 16863 as shown at dcapnconstALT 16865. (Contributed by BJ and Jim Kingdon, 24-Jun-2024.)
DECID # $/\$ $/\$

Theorem	dcapnconstALT 16865*	Decidability of real number apartness implies the existence of a certain non-constant function from real numbers to integers. A proof of dcapnconst 16864 by means of dceqnconst 16863. (Contributed by Jim Kingdon, 27-Jul-2024.) (New usage is discouraged.) (Proof modification is discouraged.)
DECID # $/\$ $/\$

Theorem	nconstwlpolem0 16866*	Lemma for nconstwlpo 16869. If all the terms of the series are zero, so is their sum. (Contributed by Jim Kingdon, 26-Jul-2024.)


Theorem	nconstwlpolemgt0 16867*	Lemma for nconstwlpo 16869. If one of the terms of series is positive, so is the sum. (Contributed by Jim Kingdon, 26-Jul-2024.)


Theorem	nconstwlpolem 16868*	Lemma for nconstwlpo 16869. (Contributed by Jim Kingdon, 23-Jul-2024.)
$/\$ $\/$

Theorem	nconstwlpo 16869*	Existence of a certain non-constant function from reals to integers implies WOmni (the Weak Limited Principle of Omniscience or WLPO). Based on Exercise 11.6(ii) of [HoTT], p. (varies). (Contributed by BJ and Jim Kingdon, 22-Jul-2024.)
$/\$ WOmni

Theorem	neapmkvlem 16870*	Lemma for neapmkv 16871. The result, with a few hypotheses broken out for convenience. (Contributed by Jim Kingdon, 25-Jun-2024.)
$/\$ #

Theorem	neapmkv 16871*	If negated equality for real numbers implies apartness, Markov's Principle follows. Exercise 11.10 of [HoTT], p. (varies). (Contributed by Jim Kingdon, 24-Jun-2024.)
# Markov

Theorem	neap0mkv 16872*	The analytic Markov principle can be expressed either with two arbitrary real numbers, or one arbitrary number and zero. (Contributed by Jim Kingdon, 23-Feb-2025.)
# #

Theorem	ltlenmkv 16873*	If can be expressed as holding exactly when holds and the values are not equal, then the analytic Markov's Principle applies. (To get the regular Markov's Principle, combine with neapmkv 16871). (Contributed by Jim Kingdon, 23-Feb-2025.)
$/\$ #

14.4.8 Supremum and infimum

Theorem	supfz 16874	The supremum of a finite sequence of integers. (Contributed by Scott Fenton, 8-Aug-2013.) (Revised by Jim Kingdon, 15-Oct-2022.)


Theorem	inffz 16875	The infimum of a finite sequence of integers. (Contributed by Scott Fenton, 8-Aug-2013.) (Revised by Jim Kingdon, 15-Oct-2022.)
inf

14.4.9 Circle constant

Theorem	taupi 16876	Relationship between and . This can be seen as connecting the ratio of a circle's circumference to its radius and the ratio of a circle's circumference to its diameter. (Contributed by Jim Kingdon, 19-Feb-2019.) (Revised by AV, 1-Oct-2020.)


14.4.10 Finite group sum over unordered finite set

Syntax	cgfsu 16877	Extend class notation to include finite group sum over unordered finite set.
_gf

Definition	df-gfsum 16878*	Define the finite group sum (iterated sum) over an unordered finite set. As currently defined, df-igsum 13489 is indexed by consecutive integers, but in the case of a commutative monoid, the order of the sum doesn't matter and we can define a sum indexed by any finite set without needing to specify an order. (Contributed by Jim Kingdon, 23-Mar-2026.)
_gf CMnd $/\$ ♯ $/\$ _g

Theorem	gfsumval 16879	Value of the finite group sum over an unordered finite set. (Contributed by Jim Kingdon, 24-Mar-2026.)
CMnd ♯ _gf _g

Theorem	gsumgfsum1 16880	On an integer range starting at one, _g and _gf agree. (Contributed by Jim Kingdon, 25-Mar-2026.)
CMnd _g _gf

Theorem	gfsum0 16881	An empty finite group sum is the identity. (Contributed by Jim Kingdon, 26-Mar-2026.)
CMnd _gf

Theorem	gsumgfsumlem 16882*	Shifting the indexes of a group sum indexed by consecutive integers. (Contributed by Jim Kingdon, 26-Mar-2026.)
CMnd _g _g

Theorem	gsumgfsum 16883	On an integer range, _g and _gf agree. (Contributed by Jim Kingdon, 25-Mar-2026.)
CMnd _g _gf

Theorem	gfsumsn 16884*	Group sum of a singleton. (Contributed by Jim Kingdon, 2-Apr-2026.)
CMnd $/\$ $/\$ _gf

Theorem	gfsump1 16885	Splitting off one element from a finite group sum. This would typically used in a proof by induction. (Contributed by Jim Kingdon, 3-Apr-2026.)
CMnd _gf _gf

Theorem	gfsumz 16886*	Value of a finite group sum over the zero element. (Contributed by Jim Kingdon, 24-May-2026.)
CMnd $/\$ _gf

Theorem	gfsumcl 16887	Closure of a finite group sum. (Contributed by Jim Kingdon, 8-Apr-2026.)
CMnd _gf

14.5 Mathbox for Mykola Mostovenko

Theorem	ax1hfs 16888	Heyting's formal system Axiom #1 from [Heyting] p. 127. (Contributed by MM, 11-Aug-2018.)
$/\$

14.6 Mathbox for David A. Wheeler

14.6.1 Testable propositions

Theorem	dftest 16889	A proposition is testable iff its negative or double-negative is true. See Chapter 2 [Moschovakis] p. 2. We do not formally define testability with a new token, but instead use DECID before the formula in question. For example, DECID corresponds to " is testable". (Contributed by David A. Wheeler, 13-Aug-2018.) For statements about testable propositions, search for the keyword "testable" in the comments of statements, for instance using the Metamath command "MM> SEARCH * "testable" / COMMENTS". (New usage is discouraged.)
DECID $\/$

14.6.2 Allsome quantifier These are definitions and proofs involving an experimental "allsome" quantifier (aka "all some"). In informal language, statements like "All Martians are green" imply that there is at least one Martian. But it's easy to mistranslate informal language into formal notations because similar statements like do not imply that is ever true, leading to vacuous truths. Some systems include a mechanism to counter this, e.g., PVS allows types to be appended with "+" to declare that they are nonempty. This section presents a different solution to the same problem. The "allsome" quantifier expressly includes the notion of both "all" and "there exists at least one" (aka some), and is defined to make it easier to more directly express both notions. The hope is that if a quantifier more directly expresses this concept, it will be used instead and reduce the risk of creating formal expressions that look okay but in fact are mistranslations. The term "allsome" was chosen because it's short, easy to say, and clearly hints at the two concepts it combines. I do not expect this to be used much in metamath, because in metamath there's a general policy of avoiding the use of new definitions unless there are very strong reasons to do so. Instead, my goal is to rigorously define this quantifier and demonstrate a few basic properties of it. The syntax allows two forms that look like they would be problematic, but they are fine. When applied to a top-level implication we allow ! , and when restricted (applied to a class) we allow ! . The first symbol after the setvar variable must always be if it is the form applied to a class, and since cannot begin a wff, it is unambiguous. The looks like it would be a problem because or might include implications, but any implication arrow within any wff must be surrounded by parentheses, so only the implication arrow of ! can follow the wff. The implication syntax would work fine without the parentheses, but I added the parentheses because it makes things clearer inside larger complex expressions, and it's also more consistent with the rest of the syntax. For more, see "The Allsome Quantifier" by David A. Wheeler at https://dwheeler.com/essays/allsome.html I hope that others will eventually agree that allsome is awesome.

Syntax	walsi 16890	Extend wff definition to include "all some" applied to a top-level implication, which means is true whenever is true, and there is at least least one where is true. (Contributed by David A. Wheeler, 20-Oct-2018.)
!

Syntax	walsc 16891	Extend wff definition to include "all some" applied to a class, which means is true for all in , and there is at least one in . (Contributed by David A. Wheeler, 20-Oct-2018.)
!

Definition	df-alsi 16892	Define "all some" applied to a top-level implication, which means is true whenever is true and there is at least one where is true. (Contributed by David A. Wheeler, 20-Oct-2018.)
! $/\$

Definition	df-alsc 16893	Define "all some" applied to a class, which means is true for all in and there is at least one in . (Contributed by David A. Wheeler, 20-Oct-2018.)
! $/\$

Theorem	alsconv 16894	There is an equivalence between the two "all some" forms. (Contributed by David A. Wheeler, 22-Oct-2018.)
! !

Theorem	alsi1d 16895	Deduction rule: Given "all some" applied to a top-level inference, you can extract the "for all" part. (Contributed by David A. Wheeler, 20-Oct-2018.)
!

Theorem	alsi2d 16896	Deduction rule: Given "all some" applied to a top-level inference, you can extract the "exists" part. (Contributed by David A. Wheeler, 20-Oct-2018.)
!

Theorem	alsc1d 16897	Deduction rule: Given "all some" applied to a class, you can extract the "for all" part. (Contributed by David A. Wheeler, 20-Oct-2018.)
!

Theorem	alsc2d 16898	Deduction rule: Given "all some" applied to a class, you can extract the "there exists" part. (Contributed by David A. Wheeler, 20-Oct-2018.)
!

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