English
If s1.entries is a permutation of s2.entries, then lookup a s1 = lookup a s2.
Русский
Если entries s1 является перестановкой entries s2, то lookup a s1 = lookup a s2.
LaTeX
$$s_1.entries \sim s_2.entries \Rightarrow \mathrm{lookup}(a,s_1) = \mathrm{lookup}(a,s_2)$$
Lean4
theorem perm_lookup {a : α} {s₁ s₂ : AList β} (p : s₁.entries ~ s₂.entries) : s₁.lookup a = s₂.lookup a :=
perm_dlookup _ s₁.nodupKeys p