English
For any a,b, a ∈ (o : Part α) ↔ a ∈ o, i.e., membership is preserved under the coe from Option to Part.
Русский
Для любых a, o: a ∈ (o : Part α) ↔ a ∈ o, т.е. принадлежность сохраняется при приведение к Part.
LaTeX
$$$ {a} \in { (o : \mathrm{Part}(\alpha)) } \iff {a} \in o $$$
Lean4
theorem mem_coe {a : α} {o : Option α} : a ∈ (o : Part α) ↔ a ∈ o :=
mem_ofOption