(** As we will see in later chapters, induction on evidence is a recurring technique across many areas, and in particular when formalizing the semantics of programming languages, where many properties of interest are defined inductively. *)
(** The following exercises provide simple examples of this technique, to help you familiarize yourself with it. *)

(* Specifications: *)
(** **** Exercise: 2 stars (ev_sum)  *)
Theorem ev_sum : forall n m, ev n -> ev m -> ev (n + m).
Example reg_exp_ex2 : [1; 2] =~ App (Char 1) (Char 2).