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feat(Data/List): add lemma List.notMem_subset #37561

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feat(Data/List): add lemma List.notMem_subset #37561

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3 changes: 3 additions & 0 deletions Mathlib/Data/List/Basic.lean
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Expand Up @@ -182,6 +182,9 @@ theorem map_subset_iff {l₁ l₂ : List α} (f : α → β) (h : Injective f) :
rcases mem_map.1 (h2 (mem_map_of_mem hx)) with ⟨x', hx', hxx'⟩
cases h hxx'; exact hx'

lemma notMem_subset {l l' : List α} (h : l ⊆ l') {a : α} (ha : a ∉ l') : a ∉ l :=
fun ha' ↦ ha (h ha')

/-! ### append -/

theorem append_eq_has_append {L₁ L₂ : List α} : List.append L₁ L₂ = L₁ ++ L₂ :=
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