+lemma veq_sym (M): is_model M → symmetric … (veq M).
+/3 width=5 by veq_repl, veq_refl/ qed-.
+
+lemma veq_trans (M): is_model M → Transitive … (veq M).
+/3 width=5 by veq_repl, veq_refl/ qed-.
+
+(* Properties with extebsional equivalence **********************************)
+