Goal:
    x : nat  y : nat  H : x <= y  x0 : nat  y0 : nat  H0 : x0 = y0
  ============================
   (x + x0 <= y + y0)


DEBUG:Going to execute:
intros; Zify.zify_pre_hook; zify_elim_let; zify_op;
            Zify.zify_iter_specs