English
If a.out = b.out for erased elements a,b, then a = b; the out map is injective on Erased α.
Русский
Если a.out = b.out для стертых элементов a,b, то a = b; отображение out инъективно на Erased α.
LaTeX
$$$a.\\mathrm{out} = b.\\mathrm{out} \\Rightarrow a = b$$$
Lean4
@[ext]
theorem out_inj {α} (a b : Erased α) (h : a.out = b.out) : a = b := by simpa using congr_arg mk h