(*** at_mono *)
theorem fr2_nat_mono (f) (l):
- ∀l1. @↑❨l, f❩ ≘ l1 → ∀l2. @↑❨l, f❩ ≘ l2 → l1 = l2.
+ ∀l1. @§❨l, f❩ ≘ l1 → ∀l2. @§❨l, f❩ ≘ l2 → l1 = l2.
#f #l #l1 #H elim H -f -l -l1
[ #l #x #H <(fr2_nat_inv_empty … H) -x //
| #f #d #h #l #l1 #Hld #_ #IH #x #H