leanprover-community / leanprover-community/mathlib4

should `clear_` tactic also clear inaccessible hypotheses?

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Description

In mathlib3, pairwise_and_iff has a usage of clear_ with this context:

α : Type u_1,
R S : α → α → Prop,
l : list α,
_x : pairwise R l ∧ pairwise S l,
_fun_match : pairwise R l ∧ pairwise S l → pairwise (λ (a b : α), R a b ∧ S a b) l,
hR : pairwise R l,
hS : pairwise S l
⊢ pairwise (λ (a b : α), R a b ∧ S a b) l

(see https://github.com/leanprover-community/mathlib/blob/aff61c0f687c5d366edb248791f26fa69a4adfe2/src/data/list/pairwise.lean#L67)

A direct translation into Lean 4 leads to this context in the same place:

α : Type u_1
R : α → α → Prop
a : α
l : List α
S : α → α → Prop
: Pairwise R l ∧ Pairwise S l
hR : Pairwise R l
hS : Pairwise S l
⊢ Pairwise (fun a b => R a b ∧ S a b) l

In this case, the current version of mathlib4's clear_ won't drop the Pairwise R l ∧ Pairwise S l hypothesis, because it is anonymous inaccessible. We need to drop that hypothesis to get the later induction to work. It's easy enough to work around this problem by adapting the proof a bit, but it would be nice if we could have a direct translation. What would make most sense to me is for mathlib4's clear_ to also drop unnamed hypotheses (in addition to hypotheses whose names start with _). We could even make the behavior configurable.

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Research direction

Start with the clear_ tactic behavior and the pairwise_and_iff usage in src/data/list/pairwise.lean around line 67. Reproduce the Lean 4 context shown in the issue, then determine how inaccessible hypotheses should be handled and verify that the translated induction proceeds after the hypothesis is removed.

Written by the indexing model from the issue text.

Assessment

Domain
devtools
Issue type
Feature
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
30/100

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