∀ {α : Type u} {β : Type v} {γ : Type w} {x : BEq α} {x_1 : Hashable α} {m : Std.ExtHashMap α β} [inst : EquivBEq α]
  [inst_1 : LawfulHashable α] {f : α → β → γ},
  Std.ExtHashMap.filterMap (fun k v => some (f k v)) m = Std.ExtHashMap.map f m