Goal:
  
  A : Type
  f : nat → A
  m : nat
  n : nat
  IHm : ∀ n : nat, n < m ↔ nth_error (unfold m f) n = Some (f n)
  Hn : n < S m
  n0 : n ≠ m
  ============================
   (nth_error (unfold m f) n = Some (f n))


DEBUG:Going to execute:
case not_locked_false_eq_true; Ltac.assumption
Level 1: Backtrace:

In environment
A : Type
f : nat → A
m, n : nat
IHm : ∀ n : nat, n < m ↔ nth_error (unfold m f) n = Some (f n)
Hn : n < S m
n0 : n ≠ m
Unable to unify "Some (f n)" with "nth_error (unfold m f) n".
Level 0: Backtrace:

In environment
A : Type
f : nat → A
m, n : nat
IHm : ∀ n : nat, n < m ↔ nth_error (unfold m f) n = Some (f n)
Hn : n < S m
n0 : n ≠ m
Unable to unify "Some (f n)" with "nth_error (unfold m f) n".
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.