unfold hingeable
α : Type u
r : α → α → Prop
β : α → Type v
xs : KVChain (flip r) β
a : α
b : β a
ys : KVChain r β
hay : boundLeft a ys
⊢ hingeable xs (KVChain.cons a b ys hay) = boundLeft a xs
α : Type u
r : α → α → Prop
β : α → Type v
xs : KVChain (flip r) β
a : α
b : β a
ys : KVChain r β
hay : boundLeft a ys
⊢ List.relHead (Sigma.relOnFst r) xs.toList (KVChain.cons a b ys hay).toList = boundLeft a xs