openScoped.lean:1:7-1:14: error(lean.unknownIdentifier): Unknown identifier `epsilon`
openScoped.lean:4:2-4:24: error(lean.synthInstanceFailed): failed to synthesize instance of type class
  Decidable (f = g)

Hint: Type class instance resolution failures can be inspected with the `set_option trace.Meta.synthInstance true` command.
Classical.epsilon.{u} {α : Sort u} [h : Nonempty α] (p : α → Prop) : α
openScoped.lean:15:7-15:14: error(lean.unknownIdentifier): Unknown identifier `epsilon`
openScoped.lean:20:7-20:14: error(lean.unknownIdentifier): Unknown identifier `epsilon`