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