English
Mapping a function f through Erased α yields that the output equals f applied to the erased value: (a.map f).out = f(a.out).
Русский
Построение через Erased α сохраняет результат: (a.map f).out = f(a.out).
LaTeX
$$$(\\mathrm{Erased}.map\\ f\\ a).out = f\\ (a.out)$$$
Lean4
@[simp]
theorem map_out {α β} {f : α → β} (a : Erased α) : (a.map f).out = f a.out := by simp [map]