(* *)
(**************************************************************************)
-set "baseuri" "cic:/matita/nat/count".
-
include "nat/relevant_equations.ma".
include "nat/sigma_and_pi.ma".
include "nat/permutation.ma".
sigma (S (p+n)) f m = sigma p (\lambda x.(f ((S n) + x))) m + sigma n f m.
intros. elim p.
simplify.
-rewrite < (sym_plus n m).reflexivity.
+reflexivity.
simplify.
rewrite > assoc_plus in \vdash (? ? ? %).
rewrite < H.