-lemma is_at_dec_le: ∀f,i2,i. 𝐓❪f❫ → (∀i1. i1 + i ≤ i2 → @❪i1,f❫ ≘ i2 → ⊥) →
- Decidable (∃i1. @❪i1,f❫ ≘ i2).
-#f #i2 #i #Hf elim i -i
-[ #Ht @or_intror * /3 width=3 by at_increasing/
-| #i #IH #Ht elim (at_dec f (i2-i) i2) /3 width=2 by ex_intro, or_introl/
- #Hi2 @IH -IH #i1 #H #Hi elim (le_to_or_lt_eq … H) -H /2 width=3 by/
- #H destruct -Ht /2 width=1 by/
-]
-qed-.
-