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A tag is a keyword or label that categorizes your question with other, similar questions. Using the right tags makes it easier for others to find and answer your question.

Coq is a formal proof management system. It is often referred to as a proof assistant.
Lean is a theorem prover and programming language, based on the calculus of constructions with inductive types. For version-specific questions, also add the [lean3] or [lean4] tags.
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Lean 4 is the latest major version of the Lean theorem prover by Microsoft Research. Use this tag for questions specific to Lean 4; if it's about Lean in general, use the tag (lean).
for questions about type theories, which are formal systems to specify properties of objects.
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Questions asked by a beginner user of proof assistants. We make an effort to be extra kind to such users.
For questions regarding Agda: the programming language / proof assistant.
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for questions about dependent types, which are families of types which vary over elements of another type.
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with the proof assistant Isabelle, if your question is specific to a certain object logic please use the corresponding tag (eg. questions specific to Isabelle/HOL should also be tagged wi…
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Mathlib is Lean's Math Library. The library is hosted on the GitHub leanprover-community account. Do not use this tag for Lean the software. Do not use this tag for other math libraries of other pro…
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In terms of categorical semantics, an inductive type is a type whose interpretation is given by an initial algebra of an endofunctor. (from nLab)
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A tactic is a command or instruction for constructing a formal proof by applying a common proof technique. For questions about high-level techniques for constructing proofs, use the tag (strategy).
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if you are implementing a proof engine, proof assistant or something similar in code. Do not use this tag if you are just at the design stage or asking about design decisions of existing …
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Lean 3 is the previous version of the Lean theorem prover, and has an active "community" release. If using the final "official" release from 2019 (3.4.2) which is incompatible with [mathlib], make thi…
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A proof assistant, or interactive theorem prover, is a software tool to assist with the development of formal proofs by human-machine collaboration.
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Questions pertaining to equality in type theory (all kinds of equality are included: judgemental, propositional, observational, setoid equality, etc.) and equality reasoning in proof assistants.
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used for questions that ask for software with a certain property or goal, such as automated theorem provers for equational logic. If the question is not specifically for software consider …
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for questions about mathematical or logical foundations of proof assistants. Questions should be related in some way to proof assistants. Possible topics might include mathematical modell…
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Questions requesting papers or books in the literature on specific, narrow issues. Also consider the tag [recommendations].
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Categories are structures containing objects and arrows between them. Many mathematical structures can be viewed as objects of a category, with structure preserving morphisms as arrows. Reformulating …
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Cubical type theory is a version of homotopy type theory in which univalence is not just an axiom but a theorem, hence, since this is constructive, has “computational content”. Cubical type theory mod…
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Questions asking how to properly and efficiently use Proof Assistants.
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for questions a mathematician would feel at home answering and can be traced back to an area of mathematics.
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Use with coinductive types.
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Tag for questions about induction such as mathematical induction, structural induction or well-founded induction (Noetherian).
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In mathematics, and particularly in set theory, category theory, type theory, and the foundations of mathematics, a universe is a collection that contains all the entities one wishes to consider in a …
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For use with the proof assistant Isabelle with classical higher-order logic.
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when a question is related to specifics on how a proof assistant works. Do not use this tag for questions related to using a proof assistant.
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Set theory is the branch of mathematics that studies unordered collections of objects. Questions with this tag will involve proofs about sets or operations on them.
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Homotopy type theory is a flavor of type theory – specifically of intensional (Martin-Löf-) dependent type theory – which takes seriously the natural interpretation of identity types as formalizing pa…
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For questions regarding ATP's (Automatic Theorem Provers), which attempt to prove a theorem without any assistance. If you are using a Proof Assistant or an interactive theorem prover and are providin…
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For question about natural numbers $\Bbb N$, their properties and applications
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Use this for questions seeking an asset that give nebulous criteria rather than concrete specifics. If seeking literature such as books or papers use [tag:reference-request]. If seeking software use […
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Learning covers topics such as how to install, how to choose a proof assistant, working through tutorials. For using a proof assistant consider the usage tag along with a specific proof assistant like…
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The history of proof assistants and machine-assisted proofs.
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For questions that ask about possible improvements of a working piece of a proof. Make sure to also include the tag for the language used, and follow that tag's guidance regarding additional version-s…
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For questions about words, phrases and definitions that are specific to proof assistants.
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