(* File: Weight.v  (last edited on 27/10/2000) (c) Klaus Weich  *)
Fixpoint weight (a : form) : nat :=
match a with
| Falsum => 1
| Atom _ => 1
| AndF a b => S (weight a + weight b)
| OrF Falsum b => S (weight b)
| OrF (Atom _) b => S (weight b)
| OrF a b => S (S (weight b + weight a))
| Imp a b => weight_neg a + weight b
end
with weight_neg (a : form) : nat :=
match a with
| Falsum => 0
| Atom _ => 0
| AndF a b => S (weight_neg a + weight_neg b)
| OrF a b => S (S (S (weight_neg a + weight_neg b)))
| Imp Falsum b => 1
| Imp (Atom _) Falsum => 2
| Imp (Atom _) (Atom _) => 1
| Imp (Atom _) b => S (S (S (weight_neg b)))
| Imp a b => S (S (S (S (weight_neg b + weight a))))
end.
Fixpoint weight_goal (a : form) : nat :=
match a with
| Falsum => 0
| Atom _ => 0
| AndF _ _ => 1
| OrF _ _ => 1
| Imp _ b => S (weight_goal b)
end.
Definition weight_gamma :=
fold_right (fun (a : form) (n : nat) => weight a + n) 0.
(**********************************************************************)

Lemma weight_ge_1 : forall a : form, 1 <= weight a.
intros a; elim a; clear a.