| T_Branch a b => nd_prod (nd_id a) (nd_id b)
end.
+ Fixpoint nd_weak' (sl:Tree ??Judgment) : sl /⋯⋯/ [] :=
+ match sl as SL return SL /⋯⋯/ [] with
+ | T_Leaf None => nd_id0
+ | T_Leaf (Some x) => nd_weak x
+ | T_Branch a b => nd_prod (nd_weak' a) (nd_weak' b) ;; nd_cancelr
+ end.
+
Hint Constructors Structural.
Lemma nd_id_structural : forall sl, Structural (nd_id sl).
intros.
destruct a; auto.
Defined.
+ Lemma weak'_structural : forall a, Structural (nd_weak' a).
+ intros.
+ induction a.
+ destruct a; auto.
+ simpl.
+ auto.
+ simpl.
+ auto.
+ Qed.
+
(* An equivalence relation on proofs which is sensitive only to the logical content of the proof -- insensitive to
* structural variations *)
Class ND_Relation :=