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+Require Import List.
+Require Import ZArith.
+Require String. Open Scope string_scope.
+Ltac Case s := let c := fresh "case" in set (c := s).
+
+Set Implicit Arguments.
+Unset Strict Implicit.
+
+Inductive sv : Set :=
+| I : Z -> sv
+| S : list sv -> sv.
+
+Section sv_induction.
+
+Variables
+  (VP: sv -> Prop)
+  (LP: list sv -> Prop)
+
+  (VPint: forall n, VP (I n))
+  (VPset: forall vs, LP vs -> VP (S vs))
+  (lpcons: forall v vs, VP v -> LP vs -> LP (v::vs))
+  (lpnil: LP nil).
+
+Fixpoint setl_value_indp (x:sv) {struct x}: VP x :=
+  match x as x return VP x with
+  | I n => VPint n
+  | S vs =>
+    VPset
+    ((fix values_indp (vs:list sv) {struct vs}: (LP vs) :=
+      match vs as vs return LP vs with
+      | nil => lpnil
+      | v::vs => lpcons (setl_value_indp v) (values_indp vs)
+      end) vs)
+  end.
+End sv_induction.
+
+Inductive slt : sv -> sv -> Prop :=
+| IC : forall z, slt (I z) (I z)
+| IS : forall vs vs', slist_in vs vs' -> slt (S vs) (S vs')
+
+with sin : sv ->  list sv -> Prop :=
+| Ihd : forall s s' sv', slt s s' -> sin s (s'::sv')
+| Itl : forall s s' sv', sin s sv' -> sin s (s'::sv')
+
+with slist_in : list sv -> list sv -> Prop :=
+| Inil : forall sv',
+  slist_in nil sv'
+| Icons : forall s sv sv',
+  sin s sv' ->
+  slist_in sv sv' ->
+  slist_in (s::sv) sv'.
+
+Hint Constructors sin slt slist_in.
+
+Require Import Program.
+
+Program Fixpoint lt_dec (x y:sv) { struct x } : {slt x y}+{~slt x y} :=
+  match x with
+    | I x => 
+      match y with
+        | I y => if (Z_eq_dec x y) then in_left else in_right
+        | S ys => in_right
+      end
+    | S xs => 
+      match y with
+        | I y => in_right
+        | S ys =>
+          let fix list_in (xs ys:list sv) {struct xs} : 
+            {slist_in xs ys} + {~slist_in xs ys} :=
+            match xs with
+              | nil => in_left
+              | x::xs =>
+                let fix elem_in (ys:list sv) : {sin x ys}+{~sin x ys} :=
+                  match ys with
+                    | nil => in_right
+                    | y::ys => if lt_dec x y then in_left else if elem_in
+                      ys then in_left else in_right
+                  end
+                in 
+                if elem_in ys then 
+                  if list_in xs ys then in_left else in_right
+                else in_right
+            end
+            in if list_in xs ys then in_left else in_right
+      end
+  end.
+
+Next Obligation. apply H; inversion H0; subst; trivial. Defined.
+Next Obligation. inversion H. Defined.
+Next Obligation. inversion H. Defined.
+Next Obligation. inversion H; subst. Defined.
+Next Obligation.
+  contradict H. inversion H1; subst. assumption. 
+  contradict H0; assumption. Defined.
+Next Obligation.
+  contradict H0. inversion H1; subst. assumption. Defined.
+Next Obligation.
+  contradict H. inversion H0; subst. assumption. Defined.
+Next Obligation.
+  contradict H. inversion H0; subst; auto. Defined.
+