additions and corrections for the article on λδ-2B

[helm.git] / matita / matita / contribs / lambdadelta / static_2 / syntax / aarity.ma

diff --git a/matita/matita/contribs/lambdadelta/static_2/syntax/aarity.ma b/matita/matita/contribs/lambdadelta/static_2/syntax/aarity.ma
index 89199fabb3ea2b76f06b12b60fe976afeabb4409..db382096447ab7046a07a5e963bb64ab3cd9127c 100644 (file)
--- a/matita/matita/contribs/lambdadelta/static_2/syntax/aarity.ma
+++ b/matita/matita/contribs/lambdadelta/static_2/syntax/aarity.ma
@@ -72,3 +72,11 @@ lemma eq_aarity_dec: ∀A1,A2:aarity. Decidable (A1 = A2).
   ]
 ]
 qed-.
+
+lemma is_apear_dec (B) (X): Decidable (∃A. ②B.A = X).
+#B * [| #X #A ]
+[| elim (eq_aarity_dec X B) #HX ]
+[| /3 width=2 by ex_intro, or_introl/ ]
+@or_intror * #A #H destruct
+/2 width=1 by/
+qed-.