English
If α is finite, then any Subtype {x // p x} is finite.
Русский
Если α конечен, то любой подтип {x // p x} конечен.
LaTeX
$$$\operatorname{Finite}(\alpha) \Rightarrow \operatorname{Finite}(\{ x\mid p x\}).$$$
Lean4
/-- This instance also provides `[Finite s]` for `s : Set α`. -/
instance finite {α : Sort*} [Finite α] {p : α → Prop} : Finite { x // p x } :=
Finite.of_injective Subtype.val Subtype.coe_injective