Section LeftInduction.
Let A' (n : t) := forall m, m <= z -> n <= m -> A m.
Let left_step :=   forall n, n < z -> A (S n) -> A n.
Let left_step' :=  forall n, n <= z -> A' (S n) -> A n.
Let left_step'' := forall n, A' n <-> A' (S n).

Lemma ls'_ls'' : left_step' -> left_step''.