Ltac nosub t2 :=  destruct t2; try solve[ left; auto]; right; done.

Lemma tm_eq_dec_mutual : ((forall t1 t2 : tm, {t1 = t2} + {~ t1 = t2}) * (forall t1 t2 : brs, {t1 = t2} + {~ t1 = t2}) * (forall t1 t2 : co, {t1 = t2} + {~ t1 = t2}) * (forall t1 t2 : constraint, {t1 = t2} + {~ t1 = t2})).