simp [build_pairs, build_pairs.pairs] at in_pairs
α : Type u_1
l✝ : List α
pairs✝ : List (α × α)
h✝ : pairs✝ = build_pairs l✝
l : List α
pairs : List (α × α)
p : α × α
x : α
xs : List α
h : pairs = build_pairs (x :: xs)
in_pairs : p ∈ build_pairs (x :: xs)
⊢ p.fst ∈ x :: xs
α : Type u_1
l✝ : List α
pairs✝ : List (α × α)
h✝ : pairs✝ = build_pairs l✝
l : List α
pairs : List (α × α)
p : α × α
x : α
xs : List α
h : pairs = build_pairs (x :: xs)
in_pairs : (∃ a, a ∈ xs ∧ (x, a) = p) ∨ p ∈ build_pairs xs
⊢ p.fst ∈ x :: xs