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(* 02110-1301 USA                                                     *)
(** %\subsection*{ extras :  Inter\_intersection.v }%*)
Set Implicit Arguments.
Unset Strict Implicit.
Require Export arb_intersections.
Require Export conshdtl.
From Algebra Require Export Inter.
(** - A nice lemma relating indexed intersections with binary ones. Remember that 'indexed_intersection' takes a map from some setoid I to the powerset of A, yielding the intersection of the range. As it happens, sequences can be such maps (with I=(fin n)). So suppose we have an n-length sequence of part_sets of A If n=0, its intersection is the full subset of A For n+1, take the binary intersection inter with the n-length intersection*)
Fixpoint repeated_inter (A : Setoid) (n : nat) {struct n} :
seq n (part_set A) -> part_set A :=
match n return (seq n (part_set A) -> part_set A) with
| O => fun v : seq 0 _ => full A
| S n' => fun v : seq (S n') _ => inter (head v) (repeated_inter (Seqtl v))
end.

Lemma indexed_intersection_as_repeated_inter :
forall (n : nat) (A : Setoid) (f : seq n (part_set A)),
indexed_intersection f =' repeated_inter f in _.